Terminal Message Monitor

Abstract

A monitoring system to permit a first terminal computer on line with a central processor to monitor message traffic from the central processor to other terminals concatenated or in a multidrop mode with the first. When messages are sent through a modem to a plurality of terminal computers in a concatenated configuration a verification of message traffic is provided. Logic circuitry is provided for monitoring both transmitted and received information and providing a printout of the communication line information. The monitoring terminal, regardless of the address of the message, will store the communication line information in its memory and print out the stored information to provide a positive communication system check.

Description

This invention relates generally to a data processing system wherein a series of terminal computers is on-line, through a modem, with a central processor and more particularly to a method and apparatus for monitoring message traffic between the central processor and the terminals by utilizing an intermediate terminal in a monitor mode.

In interfacing terminal computers with a central processor it is essential to determine if each unit is receiving the correct information in order to eliminate transmission inaccuracies as a source of error. This is especially troublesome when new systems are set up and when various types of terminals are added to an existing system. In data processing systems information is usually in the form of time-spaced electrical pulses arranged in a pre-determined code representing numerals and characters. It is always possible that, due to transient voltages, line malfunctions, a failure of the equipment, or other means this data will be transferred in error. Thus it is desirable to have checking or monitoring apparatus available to trace malfunctions.

A typical on-line commercial computer application requires continued access to a centrally located data file. Teleprinters on line, at various locations, were previously used to display information received and to provide a source of input, including answers to inquires from a central processor. This process helped to identify any transmission of errors to the central processor. Central processors could work independently of these teletypewriter terminals but the flow of received and transmitted information was significantly impeded by the much slower printing operation. Since the teletype printers had no memory they significantly hampered time-sharing efforts.

An example of a system of this type would be a bank operation where it is necessary to obtain or update certain account balances kept in a file of bank accounts. Similarly insurance companies find it is necessary to periodically update information in various files such as the amount of insurance, the premium data and the extent of insurance coverage for each insured person. High speed electronic data processing systems are commonly used for storing files of information for banks, insurance companies and the like. Further in such businesses, it is often desirable to have a centrally located data processing system including high speed electronic central processor and peripheral equipment and to employ remote terminal devices for entering and receiving information at any of a number of remote points. Such remote terminals may be located, for example, at branch offices of the banks or insurance companies.

When installing these systems and adding new terminals to existing systems accurate checking and monitoring apparatus is necessary for field engineering people to solve complex interface requirements. A number of prior art techniques have proved to be expensive and excessively time consuming when applied to the new sophisticated terminals. A known method of verifying message traffic was to insert test messages into the line and read these messages at a distant point to determine accuracy of transmission. This system required external connections and interrupted data flow considerably. Also, the tests conducted were not under actual conditions, since chosen test messages were inserted.

Another approach was to provide a permanent monitoring and checking apparatus as a part of an installation. This was effective in tracing errors and signalling an operator, but required expensive permanent equipment and, obviously, increased the cost of the user. More recently field "monitoring kits" have been provide wherein known specifications are used as a standard, with lights or counters indicating variations from the norm. These devices are portable, but must be connected between the modem and a terminal and left "on" for a period of time so that intermittent errors can be recorded. The most recent development is a channel monitor which is externally connected anterior to a terminal unit. This monitor synchronizes, controls, and decodes monitored information for printing, in code, format characters. Again, this is an external system costing several thousands of dollars and requiring complex interconnections.

It has been found that transmission errors occur mainly during start-up operations and when interfacing foreign systems. Also, terminal units have been most economically set up in series, or concatenated, configuration utilizing a single modem.

It is, therefore, the primary object of this invention to provide for an improved monitoring system for verifying message traffic.

It is another object of this invention to utilize a concatenated terminal computer as a monitor for sampling both transmitted and received information.

It is another object to provide a printout of message traffic without affecting terminal or central processor operation.

It is still another object of this invention to selectively use a terminal computer as a monitor by simple and inexpensive means, so that once an error is found and corrected the monitor can quickly be converted back to a terminal unit.

BRIEF STATEMENT OF THE INVENTION

To accomplish the above objects, logic means is selectively employed to utilize the memory and computational ability of a terminal computer in a monitor mode. Gating circuitry is also provided to place both transmitted and and received information on the same line when in the monitoring mode. A terminal computer in the concatenated mode is utilized as a monitor by non-destructively storing communication line information from or to any other terminal, by use of logic circuitry and the memory of the terminal, and then printing out this information for visual review by a field engineer for aberrations in the flow of signals. External connections or systems are not required and an inexpensive modification is used to utilize the existing internal micro-logic of the terminal computer.

DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a block diagram of a data processing system of the type in which the invention may be utilized;

FIG. 2 is a symbolic block diagram of a remote terminal computer utilizable in accordance with the principles of the invention;

FIG. 3 is a simplified functional block diagram of the remote computer terminal of FIG. 2;

FIG. 4 is a flow diagram illustrating the steps performed in order to monitor data on a terminal computer; and

FIG. 5 shows logic circuitry for handling received and transmitted information through a monitoring terminal computer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is illustrated a data processing system capable of utilizing the present invention. A system of this type may be found in a banking industry wherein a central processing unit (CPU) 11 is located at a central or main bank headquarters. Operatively connected to the CPU 11 is an on-site modem 13. This modem is connected via a communications channel and customarily over a telephone communications system to a central office bridge 15. The central office bridge 15 functions to connect the CPU modem 13 to one of a plurality of receiver modems 17 and 19 located at receiver sites. A receiver site may be for example, an individual branch bank. The receiver modems 17 and 19 are connected to the central office bridge 15 by a four or two wire communication channel. Operatively and electrically connected to the receiver site modems 17 and 19, is a plurality of terminal computers (TC). The first terminal in the line, 21 or 27, is followed by a plurality of succeeding terminals 23, 25 and 29, respectively, which are electrically connected in concatenation. In an actual operation there may be a large number of receiver site modems, each with many terminal computers.

In the banking industry, for example, each terminal computer may represent a teller's window at a branch bank. Thus, from the modem 17, for example, the branch bank associcated therewith has three terminal computers on the modem line, i.e., 21, 23, and 25. Similarly, a branch bank having the modem 19 has terminal computers in line therewith, but as is illustrated, this comprises only two terminal computers, i.e., 27 and 29. As may be seen, a number of terminal computers may be connected in concatenation with a modem at a branch office. In operation, any instruction or request from the CPU 11 will contain an address representing one of the terminal computers at a particular branch bank. Actually, a two character address is provided which indicates which terminal computer in the system is being addressed. When a particular terminal computer at a teller window is addressed, it then has the responsibility to reply to the CPU.

Referring now to FIG. 2, there is shown a simplified block diagram of a remote terminal in which the principles of the present invention may be incorporated. A typical remote terminal such as 21 may be described as comprising three major sections: a main processor 33, utilizing a main memory 35 and an input/output keyboard 37; a line discipline processor 41, with its message memory 43 and terminal address control logic 45; and a terminal buffer storage 47. The output of the buffer storage 47 is coupled by modem 17 to the input of a communication line 22. The discussion of the preferred embodiment will deal only with those terminals in line with the modem 17. However, it is apparent that the explanation is applicable to the modem 19, as well, or to any group of terminal computers in concatenation with an on-line modem. The structure and operational interrelationship of the sections of the terminal 21 is discussed in detail hereinafter in conjunction with FIGS. 3, 4 and 5 with similar reference numerals being used to designate the respective sections of the terminal 21.

Now referring to FIGS. 1 and 2 in conjunction with Table I below, the format and function of the various portions of inquiry and response messages transmitted between the central processor and the remote terminals may be understood.---------------------------------------------- -----------------------------TABLE 1 A. POLL, TRANSMISSION, ACKNOWLEDGEMENT AND SIGNOFF CENTRAL DATA POLL PROCESSOR ACK. e a a p e a o d d o n c t 1 2 l q message k line discipline s a a x s e b e processor o d d m t t e x t t c o h 1 2# x x c t b. poll with no message central data poll signoff processor e a a p e o d d o n t 1 2 l q line discipline e processor o t c. selection, acceptance, message receipt and acknowledgement central data select message processor e a a s e s ae bx s o d d e n ack o dt cm t t e x t ack t 1 2 l q h 1x c x a a line discipline c c processor k k d. selection with no acceptance central data select processor e a a s e o d d e n neg. t 1 2 l q ack. line discipline n processor a k legend function message format 1 2 3 . . . . . _________________________________________________________________________ _ .

entries A through D of Table I illustrate typical examples of messages exchanged between the remote terminal such as 21 or 23 and the central processor 11. A remote terminal is capable of operating in either an off-line or an on-line mode. In the off-line mode, processing tasks are accomplished by the main processor 33 in accordance with program and object data stored in its memory 35. In the on-line mode, the main processor 33 preferably operating in a poll or select mode, relies upon communications with the central processor 11 over the communication line for at least a portion of its operation.

A poll inquiry message is defined as a message by which the central data processor 11 interrogates one of a plurality of the addressed remote terminals and inquires whether the addressed remote terminal has a message ready for transmission to the central processor. A select inquiry message is defined as a message by which the central data processor interrogates one of a plurality of the addressed remote terminals in the communication net informing the addressed remote terminal that the central processor has a message ready for transmission to the addressed remote terminal.

In either the poll or select inquiry mode, if an addressed remote terminal is not ready to receive a message, i.e., it is either being operated off-line or it is otherwise not ready to receive or transmit a message in response to the received inquiry message addressed to it, the line discipline processor 41 automatically responds with an appropriate "not ready" message to the central processor 11. As shown in entries B and D of Table I, the line discipline processor 41 responds to a poll with an EOT (End of Transmission) and to a select with a NAK to indicate that it is not ready to transmit or receive a message, respectively. Upon receiving a negative acknowledgement from the addressed remote processor, the central processor either retransmits its message, which may have been garbled in the transmission channel, or it may continue on its poll or select sequence to the next remote terminal in the normal addressing sequence.

Referring to entries A through D of Table I, the function or explanation of the message is written above the signal indicating waveform-type line and the message format is indicated below the line. The message format includes, reading left to right, characters 1, 2, 3 . . . N. The respective characters indicated are those of the United States of America Standard Code for Information Interchange (USASCII).

Entry A of Table I illustrates a message exchange for a typical poll operation. The first character in the message transmitted by the central data processor 11 comprises an end of transmission character EOT. All transmissions may begin with this EOT character or another suitable character. Following the EOT character are two address characters AD1 and AD2. In a typical multi-terminal line environment each remote terminal, be it 21, 23, or 25, would have assigned to it a plurality of addresses indicating characters which are, for example, stored in an expected message portion of the memory of the line discipline processor 41. Following the address characters are the POL (poll) and ENQ (inquiry) characters. Entries C and D of Table I show a similar message from the central data processor 11 as assembled and transmitted for the selection-type inquiry message with the SEL (select) character replacing the POL character of the poll message format illustrated in entries A and B.

Referring again to FIGS. 1 and 2 it may be seen that each terminal 21, 23 or 25 through its line discipline processor 41 responds only to the messages specifically addressed to that terminal even though the communication network is in a multi-drop mode. In the multi-drop mode each terminal 21, 23, or 25 receives all messages transmitted by the central processor. Each terminal is in effect a slave to the central processor which establishes the poll and select routing and the frequency with which each respective terminal 21, 23, or 25 in the net will receive a poll or select message addressed to it.

The terminal address control logic 45, which may comprise a shift register or other memory store and logic gating for storing an address generated either by the arithmetic unit of the main processor 33 or of line discipline processor 41, expands the capabilities of the remote terminal by permitting either the terminal operator or the central processor to determine in advance the address of the terminal. This in turn determines which subsequently received messages the remote terminal will respond to by determining when the present, i.e., altered, address of the remote terminal corresponds with the message address characters of the inquiry message transmitted from the central processor. As hereinafter is more fully explained in conjunction with FIGS. 3, 4, and 5, by selectively changing a remote terminal, such as terminal 21, into a message monitor, any comparison with an expected message store is bypassed so that all data coming through the modem 17 will appear to be addressed to the terminal 21 but also be acted on by the addressed terminal.

Referring now to FIG. 3, there is illustrated a logic block diagram of the remote computer terminal 21 capable of modification to a monitor. As hereinabove described the remote terminal 21 comprises three major sections: a main processor 33, a line discipline processor 41 and a terminal buffer storage 47.

The main processor 33 preferably comprises a stored program machine in which object data is manipulated in an arithmetic unit 51 in accordance with a sequence of micro-logic instructions stored in and withdrawn from the main memory 35 in a predetermined sequence. The input channel 36 and the keyboard 37 are arranged to selectively enter programs and object data into the processor 33 via an input buffer register 39. The main memory 35 may comprise, for example, a rotatable magnetic disk having a plurality of read/write heads for accessing an unrestricted general memory section and a plurality of read only heads for accessing a restricted stored program portion of the memory. The information and object data stored in the main memory is processed in the arithmetic unit 51 which may include, for example, a full adder and appropriate input gating selection networks, not shown. A memory address register (MAR) 53 is operatively associated with the memory select matrix 35 via gates 55 and 57 to access an appropriate portion of the memory data in response to an address loaded in the MAR by an instruction decoder 59.

In operation of the main processor in accomplishing its tasks as designated by a given program being run, the memory address register 53 periodically addresses and interrogates the main memory 35 and withdraws therefrom an appropriate program instruction indicated by the address located by the instruction decoder 59 into the memory address register 53. The micro-logic instructions withdrawn from the read only portion of memory 35 are sequentially loaded into the instruction decoder 59. The output of the instruction decoder 59 enables appropriate control logic for controlling various gating functions in the main processor in accordance with the contents of the instruction decoder 59. The instruction decoder, in response to withdrawn program instructions, controls a state machine 61 via a gate 63. The state machine 61, which may comprise a counter, generates a sequence of timed machine state levels or timing pulses for controlling the various logic functions of the main processor including, for example, the operation of an adder or the exchange of information between the memory, the main processor, the instruction decoder or a printer 65. As shown, either the arithmetic unit 51 or the memory 35 of the terminal processor in conjunction with the instruction decoder 59 may directly actuate the printer 65 via gates 67 and 69, respectively, thus providing a hard copy of the results of the arithmetic units computation.

As in a normal stored program machine, after each instruction is decoded by the instruction decoder 59 and executed by the memory 35 and state machine 61, the processor, for example through its adder logic, generates an advance signal to increment, for example, a counter associated with the instruction decoder thereby advancing the instruction counter to the next count in its orderly count sequence. In response to the new contents of the instruction counter, the next in a series of micro-logic instructions would be withdrawn from the main memory 35 and serially fed to the instruction decoder 59. In this manner, the respective sequential syllables of a memory word of a program instruction would be transferred to the instruction decoder to properly energize the control matrix for withdrawing the appropriate program steps and/or data from the memory. Thereafter the instruction decoder in response to the decoded program instruction would appropriately energize the state machine 61 to generate appropriate logic timing signals to enable the main processor to accomplish the task indicated by each decoded program instruction. As the various steps of the serial program are sequentially executed an appropriate output is generated on the printer 65.

In addition to being able to operate off-line, the remote terminal 21 is capable of operating on-line and communicating with the central processor 11 as shown in FIG. 1. This communication with the central processor is controlled by the line-discipline processor 41. The line discipline processor 41 is preferably a stored program machine and may be similar in structure and operation to the main processor 33. The function of the line discipline processor is to establish line discipline in accordance with stored micro-logic for controlling the assembly, editing, formatting and parity generation-check of messages to be transitted to, and as received from, the central processor 11 for the main processor 33.

The line discipline processor 41 (FIG. 3) is preferably similar in structure and operation to the main processor 33. An auxiliary or message memory 43 is arranged to store messages to be sent to, and received from the central data processor 11 and to store a series of micro-logic instructions for controlling the operation of a data comm processor 75 of the line discipline processor 41. The message memory 43 may, for example, comprise a rotatable memory having a read-write portion for storing messages and a read-only memory for storing micro-logic instructions. A head selection matrix, not shown, which is responsive to the contents of a memory address register 77 is used to control the accessing of the message memory 43 for withdrawing micro-logic instructions and messages stored therein. The memory address register 77 of the line discipline processor 41 controls the access to, and reading of the micro-logic instructions from the message memory 43 to a decode register 79. The micro-logic instructions withdrawn from the message memory 43 are decoded in the decode register 79 with the output of the decode register 79 controlling a state machine 81 in accordance with the contents of the decoded program step. In this manner the decode register 79 controls the generation of appropriate logic gating signals for controlling the operation of the data comm processor 75, which may comprise a full adder and appropriate gating for manipulating data in accordance with decoded micro-logic instructions.

As is known in the art, the state machine 81 generates appropriate timing signals to control the operation of logic gates for example 85, 87 and 89 which control the exchange of information between the decode register 79 and the data comm processor 75. The operation of the state machine 81, which may comprise a counter, may be further controlled by appropriate control signals designated T.sub.x and R.sub.x which designate a function of the transmit or receive state of the line discipline processor 41 and buffer 47.

When the main processor 33 has a message to be transmitted to the central processor, the message to be transmitted is originally assembled by the main processor 33 in a specific area of the memory 35. After monitoring and determining the condition of transmit and receive flag registers 91 and 93 respectively, the main processor selects an appropriate time and transfers the message from memory 35 of the main processor 33 to the message memory 43 of the line discipline processor 41 for example via the arithmetic unit 51 and the decoder 59. Thereafter the main processor is free to return to its off-line task, and the line discipline processor 41 awaits the receipt of a poll from the central processor to initiate the transmission of the message stored in the message memory 43. The sequence and format of inquiry and response messages transmitted between the line discipline processor 41 and the central data processor 11 have been discussed hereinabove in detail in conjunction with Table I and FIGS. 1, 2 and 3.

When the line discipline processor 41 receives an inquiry message from the central processor 11, the appropriate R.sub.x signal, i.e., a signal for example, signifying communication line carrier detect, actuates a gate 95 thereby initiating the operation of the state machine 81 in the receive mode. The received inquiry message is transferred bit serially from the modem 17 to the buffer storage 47 as it is received serially from the line. The information stored in the buffer 47 is then compared in a comparator 101 with an expected message format previously stored, for example, in a message store 103, which may comprise any memory, for example, an array of flip flops arranged to store encoded information in the form of the expected message format as hereinabove described in conjunction with Table I.

As shown, a pair of logic gates 105 and 107 in conjunction with suitable timing signals, for example t.sub.o through t.sub.5, may be employed to transfer or couple the contents of the respective stages of the buffer storage 47 and the expected message store 103 to the comparator 101. In this manner the respective binary bits of the appropriate portions of the received message and the expected message store may be compared bit by bit to check and determine the equivalence therebetween. Also, the respective bits of the received message comprising the parity bit and the address bits may be compared to determine whether parity of the received message checks and whether the message as received is addressed to the receiving terminal.

In the event the parity and address portions of the received message compare with that information or data stored in the expected message store, the output of the comparator 101 would be logically true and a gate 109 would appropriately signal the data comm processor 75 of the line discipline processor 41. In response to this indication of comparison, the data comm processor 75 of the line discipline processor appropriately sets the memory address register 77 to withdraw from the message memory 43 an appropriate positive acknowledgement if it was determined that the line discipline processor 41 is properly conditioned to respond to the inquiry message, i.e., either a poll or select inquiry. Thereafter the message to be transmitted to the central processor 11 may be read from the message memory 43 via the logic gate 89 with an appropriate timing signal to the buffer store 47 for transmission to the central processor 11 via the communication line 22.

In connection with the above description, it is to be understood that only the remote terminal to which a particular inquiry message is addressed such as 21 responds by sending a positive or negative acknowledgement to the central processor via the communication line. In this manner the central processor 11 is able to establish and maintain a sequence of inquiry messages thereby providing orderly data transmission.

In accordance with the principles of this invention, the terminal 21 for example, is to be modified to become a monitor for message traffic from the central processor 11 to the terminals 23 or 25 as well as from the terminals 23 or 25 to the central processor. Referring to FIG. 3 and the previous description, the terminal 21, once it determines that a message is not addressed to it will not perform an operation on this message nor send a positive or negative acknowledgement signal in the normal operation of the system. Consistent with the objects of this invention, the terminal 21 is to be transformed into a monitor such that all messages regardless of their address will be monitored, but still not acted on. The terminal being used as a monitor will not function in a terminal computer mode. As has been previously discussed, all terminals on line with the same modem whether serially in a single group or in a plurality of parallel groups, i.e., in the multi-drop mode receive all messages from the central processor 11 but will act only on those messages having their address. In the following discussion, the terminal 21 is modified as a monitor, and the terminal 23 which is downstream from the terminal 21, will operate in the normal manner as a transmitter and receiver of messages.

Referring to the flow diagram of FIG. 4, all memory buffers of the terminal 21 will be cleared starting with the main memory buffer (not shown) and proceeding to the buffer storage 47. A portion of the message memory 43 is used for micro-logic instructions utilized in normal machine operation and for example, a message memory of at least 256 words may normally have only 64 words available for line traffic storage. However, in a monitor situation, 192 words are provided for use by the terminal computer for line traffic storage. This operation is referred to as redesignation, in the flow diagram.

In the monitor mode, the gate 109, in FIG. 3, will signal the data comm processor 75 that a message has been received and the comparison with the expected message store is positive. This is accomplished, even though the message is not addressed to the monitor terminal 21, by suppressing the comparison in comparator 10 thereby providing a logically true reading at the input to gate 109. A monitor mode gate 160 is provided between the gate 105 and the comparator logic 101 to couple messages from the buffer storage 47 directly through gate 109 to the message memory 43 when the monitor mode is in operation thus providing an appropriate signal at the other input of the gate 160. This step is indicated as suppress comparison on the flow chart of FIG. 4.

Contrary to normal machine operation, a response withdrawn from the message memory 43 by memory address register 77 giving an appropriate positive acknowledgement to a received message is undesirable, since the terminal 21 is now performing a monitoring function. Any response to message inquiries is suppressed by means of a monitor mode input switch 163 which is normally supplying a ground to one input of the gate 89. During the monitoring mode, this switch is opened thereby inhibiting any output to the central processor 11 through the gate 89.

All line traffic data including noise signals, for the terminals served by modem 17 will now be received by the buffer storage 47 of the monitor and transferred to the message memory 43. When the message memory is full, or at any other time the monitoring operator may desire after at least one character of data appears on the communication line, an interrupt signal can be initiated. This may be accompolished by depressing an interrupt key on the keyboard 37. Storage of data will cease and the contents of the message memory will be transferred into the main memory 35 and then printed in the sequence that each character appeared on the communication line.

Transmitted information from the terminal 23, in our example, presents special monitoring problems which are solved with the present invention. Transmitted information in the context of this application refers to data transmitted from a downstream terminal, one other than the monitoring terminal, to the central processor. Referring now to FIG. 5, there is shown logic circuitry for handling received and transmitted information through the monitoring terminal computer. Circuit modifications are necessary to monitor transmission when terminal 21 is in the monitoring mode. Transmitted information from "downstream terminals" is, normally, not sampled for address checks by the terminal 21. The logic circuitry disclosed performs a routing function by transferring the transmitted data in (TRDATAI) from "downstream terminals" to the received data line (RCDATA), as well as providing a variety of alternate modes. More specifically, a signal transmitted from a downstream terminal will enter a NAND gate 130, provided for signal isolation, and then to an amplifier 131 to restore its signal level. This message from the terminal 23, in our example, will then proceed to be transmitted (TRDATAO) through the modem 17 and to the central processor.

Data transmitted from a downstream terminal will also be routed through a pair of NAND gates 132 and 133 to enter the terminal 21 received data line (RCDATA). The NAND gates, throughout the system, perform an isolating function to prevent signal interference as well as providing for proper signal inversion. The transmitted data also enters a NAND gate 134 the output of which is one input into a three-input NAND gate 135. Receive data input (RCDATAI) from the modem 17 is fed through a NAND gate 138, again for isolation, and a second NAND gate 139 to the RCDATA line. The RCDATAI is also fed through a NAND gate 136 and through an amplifier 137 to the next downstream terminal.

Request to send input signal (REQSNDI) which indicates that a downstream terminal is requesting to send information, travels directly to the modem, to turn on the communication line carrier. This REQSNDI signal is used in monitor 21 to inhibit the received data in line (RCDATAI) when a downstream terminal is requesting to send a message. This REQSNDI signal is then fed through a pair of NAND gates 141 and 142 which are connected in series. From node C4, which is located between the output of NAND gate 138 and the input of NAND gate 139, the signal is then fed into the input of NAND gate 139 permitting the RCDATA line to be controlled by the gate 133.

As can be seen, both transmit and received data are fed to terminal computer 21 on its RCDATA line, thereby allowing the monitoring of both transmit and receive messages but inhibiting RCDATAI when the REQSNDI line is active. By the use of several switching points various data lines may be isolated and sampled to solve particular machine problems. For example, a switching point I4 is located between the output of NAND gate 141 and the input to NAND gate 142, and a switching point M4 is located between the output of NAND gate 132 and the input of NAND gate 133. Ground is provided at node C.sub.4 and each of the described points and denominated, respectively, C3, I3, and M3. To monitor both the received and transmit lines simultaneously, point I4 may be connected to ground I3 to provide grounding of the REQSNDI signal in the monitor terminal. To monitor the RCDATAI line and inhibit the TRDATAI line when the request to send is active, switching point M4 may be connected to ground M3 thereby grounding their respective signals. To monitor TRDATAI only, node C4 may be connected to ground C3 thereby inhibiting RCDATAI. To monitor RCDATAI only, connect switching point M4 to ground M3 and switching I4 to ground I3 thereby inhibiting TRDATAI and REQSNDI signal in the monitor terminal. By the proper interconnection, other data lines and combinations may be tested.

A signal from moden 17 appearing on data carrier detector line (DCARDETI) indicates that the modem is operating correctly and is "on". Line DCARDETI is also one of the inputs to the NAND gate 135 with RCDATAI providing the third input at the NAND gate 135. The first input of gate 135, as previously described, was that of transmitted data in line TRDATAI. When all three signal appear at the input of the gate 135 indicating that data is appearing on both the RCDATAI line and the TRDATAI line at the same time, a flip-flop 143 will be set registering an error condition. When the error condition has been noted and/or corrected, the flip-flop 143 can be reset by momentarily grounding an input E4 of flip-flop 143 to ground E3.

As is apparent from the above-discussion, complete versatility for testing data flow is provided to the field engineer by relatively simple gating and logic functions. Terminal interfacing and error detection are thus enhanced at a minimum of cost and external equipment.

Claims

1. In a system wherein a central data source communicates messages with a plurality of concatenated remote terminals, each of said terminals normally comparing a received message address from said central source with its terminal address to exclude messages to other terminals, a method for temporarily utilizing one of said terminals having memory means and display means for monitoring terminals more remote from the central data source, comprising the steps of: clearing a portion of said monitoring remote terminal memory means for temporarily storing said messages received from said central data source, suppressing said address comparison in said monitoring terminal to allow all received messages, regardless of their address, to enter said monitoring terminal, temporarily storing said received messages in said cleared portion of said terminal memory means, and displaying said temporarily stored messages in response to an inquiry

2. The monitoring method according to claim 1 additionally including the step of: gating messages transmitted by said more remote terminals to said

3. The monitoring method according to claim 2 additionally including the step of: selectively isolating the monitoring of messages received from said central

4. The monitoring method according to claim 2 additionally including the step of: registering the concurrent monitoring of messages received from said control data source and messages transmitted from said more remote

5. In a system wherein a central data source communicates with a plurality of concatenated remote terminals, each of said terminals having means for normally comparing the address of a message received from said central source with its terminal address to exclude messages to other terminals, apparatus for temporarily converting one of said terminals having memory means and display means to a monitor as to terminals more remote from said central data source comprising: means for suppressing said comparison means and for routing all messages received from said central data source, regardless of address, to said memory of said monitoring terminal, means for gating messages transmitted from said more remote terminals to said monitoring terminal memory, and means for selectively isolating said suppressing and routing means and said

6. In a system wherein a central source communicates data through a selected remote terminal, having memory means and display means, and being serially connected to at least one other more remote terminal, each terminal normally comparing the address of a received message from said central source with its terminal address to exclude messages to other terminals, a method for temporarily utilizing said selected remote terminal for monitoring communications between said central source and said more remote terminal comprising the steps of: clearing a portion of said memory means of said selected remote terminal for temporarily storing said received message; suppressing said address comparison in said selected remote terminal to allow all received messages, regardless of their address, to enter said selected remote terminal; storing said received message temporarily in said cleared portion of said memory means; and displaying said temporarily stored message in response to an inquiry command.