U.S. patent number 3,569,943 [Application Number 04/812,812] was granted by the patent office on 1971-03-09 for variable speed line adapter.
This patent grant is currently assigned to International Business Machines Corporation, Armonk, NY. Invention is credited to David Mackie, Eugene E. Mallar, Jr., Robert F. Steen.
United States Patent |
3,569,943 |
|
March 9, 1971 |
VARIABLE SPEED LINE ADAPTER
Abstract
A variable speed line adapter for receiving start-stop data
transmission is normally set for reception at one speed and will
examine the starting characters of received data at that speed. If
the first received character is not a complete and valid starting
character for the initial receiver speed, a shift is made to
another receiver speed and the next data character examined. By a
proper selection of starting characters, the adapter may be
switched from one speed range to another until the line adapter is
set to receive at the speed of the transmitted signal.
Inventors: |
David Mackie (Raleigh, NC),
Eugene E. Mallar, Jr. (Raleigh, NC), Robert F. Steen
(Raleigh, NC) |
Assignee: |
International Business Machines
Corporation, Armonk, NY (N/A)
|
Family
ID: |
25210698 |
Appl.
No.: |
04/812,812 |
Filed: |
April 2, 1969 |
Current U.S.
Class: |
705/22;
710/8 |
Current CPC
Class: |
G06F
13/385 (20130101); G06Q 20/203 (20130101) |
Current International
Class: |
G06F
13/38 (20060101); G05b 011/00 (); G06c
009/00 () |
Field of
Search: |
;340/172.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Paul J. Henon
Assistant Examiner: Paul R. Woods
Attorney, Agent or Firm: Hanifin and Jancin Delbert C.
Thomas
Claims
1. In a line adapter for communication systems of the start-stop
type wherein a character is represented by a signal having a
plurality of selectively coded bits preceded by a start bit and
followed by a stop bit and wherein terminals transmitting character
signals at different speeds may be selectively connected to a line
adapter, the combination of: a plurality of oscillators, one for
each of said different speeds; gating means to pass the output of
each of said oscillators to a common line adapter control lead;
signal decoding means initiated by reception of said start bit and
controlled by the oscillator signals on said control lead;
initializing means to set said gating means to pass a predetermined
one of said oscillator outputs to said control lead; character
control devices to indicate when a stop bit should be present in a
received character transmitted at the transmission rate of said
predetermined oscillator output; and a gating means control circuit
activated by said character control devices to prevent alteration
of said gating means if said received character signal is at a stop
bit level or to alter said gating means to pass the output of a
different oscillator to said control lead if said character
2. A line adapter as recited in claim 1 wherein: said gating means
comprises a plural state circuit; said gating means control circuit
comprises a bistable device initially set to one stable state; a
circuit gated by said bistable device when in said one state to
change the state of said plural state circuit if the received
character signal does not have a stop bit when sampled by said
character control devices; and another circuit to set said bistable
device to its other state if the
3. In a line adapter for communications of the start-stop type in
which a character is represented as a predetermined number of
selectively variable bits preceded by a start bit and followed by a
stop bit and wherein any one of a plurality of terminals
transmitting at different rates may be connected to a line adapter,
the combination of: a plurality of adapter timing circuits, one for
each of said different rates; gating means to pass the timing
signals on one of said timing circuits to a common timing lead; an
adapter initiating means to set said gating means to pass a
predetermined one of said timing signals to said timing lead; a
transmission rate detecting device set by said initiating means and
activated by reception of a start bit of a character to enable
selection of one of said timing signals for control of said line
adapter by indicating the time for reception of the stop bit of a
character transmitted at the rate corresponding to said
predetermined one of said timing signals; means under control of
said initiating means and said rate detecting device to test for
the occurrence of a stop bit; and gating control means activated
when said controlled means does not detect a stop bit to set said
gating means to pass the timing signals corresponding to the
transmission rate of the character representations being received.
Description
OBJECTS OF THE INVENTION
Line adapters are well-known communications devices and are
installed between a commercial data set and a data processor. A
line adapter functions in data reception to sample the line voltage
levels as provided by the data set and to supply the samples to the
data processor with the required signal characteristics. In data
transmission, the line adapter will function to convert the signals
from the processor into data set controlling pulses. In both of
such functions, it is necessary that the line adapter be operated
at the correct speed so as to synchronize with incoming signals and
to provide outgoing signals at the proper speed for the terminals
connected to the transmission line.
In private wire systems, a terminal can be permanently connected to
the main processor through a line adapter which will always operate
at a single speed. If, however, a system includes a plurality of
terminals of mixed transmission speeds, each of which can be
connected to the processor over the commercial switched telephone
network, it has heretofore been necessary to restrict each terminal
to connection to only those line adapters which are operating at
its transmission speed. Such a restriction forces a complicated
arrangement for the system which must provide sufficient line
terminals at each transmission speed to accommodate the expected
traffic at that speed, effectively determining a separate
transmission system for each speed rather than a single system as
required for the total expected traffic.
By providing line adapters each of which will automatically adjust
itself for reception of any speed of transmission within the system
and which can be adjusted to send at any transmission speed used
within the system, a simplification of the system and a reduction
in the number of transmission lines can be effected. In such an
adapter, there is no need for more line adapters than is required
by the overall traffic load and a less expensive, more efficient
transmission network results.
It is then an object of this invention to provide a variable speed
line adapter for a start-stop transmission system to enable
selective connection of terminals of different transmission rates
to said line adapter.
It is also an object to develop a line adapter responsive to
predetermined signal combinations transmitted from a terminal to
adjust itself into synchronism with the transmission speed of said
terminal.
Another object is to provide a line adapter having clock control
circuits to adjust the clock speed of the adapter to one of a
plurality of discrete rates under control of preliminary characters
transmitted to said adapter from any one of a plurality of
transmitting terminals having different rates.
A still further object is to provide a line adapter with clock
control circuits responsive to reception of predetermined control
characters to set said line adapter clock to one of a plurality of
discrete operating speeds.
The foregoing and other objects, features and advantages of the
invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawing.
DRAWINGS
FIG. 1 shows a part of the pertinent circuits of a line
adapter.
FIG. 2 shows the remainder of the pertinent line adapter circuits
and an oscillator control to pass a selected oscillator output to
the line adapter.
FIG. 3 is a diagram of the timing between two received
characters.
FIG. 4 is a modification of the oscillator control circuits of FIG.
2 when more than two speeds of transmission are received.
FIG. 5 is another embodiment for selecting a proper one of a
plurality of oscillator speeds.
DESCRIPTION OF THE INVENTION
The line adapter of this invention is herein described in a
configuration which it would have if connected to a transmission
control unit such as the commercial IBM Model 2702,one model of
which is set out in the Richard et al. U.S. Pat. No. 3,337,855
issued on Aug. 22, 1967. The present line adapter could be used as
one of the line adapters 54 shown in FIG. 1 of the patent. The line
adapter will, in a start-stop system, be normally inactive with the
transmission line at a mark level, i.e., with a significant voltage
thereon. When the line drops to a space level the line adapter
starts operation and sends successive samples of the line voltage
to the attached transmission control unit (TCU) for assembly into a
character. When a character has been assembled in the TCU, it sends
a stop signal, see the bottom of FIG. 47 of the above Richard et
al. patent, to terminate operations in the line adapter until
another start signal, i.e., space, is received.
In the pertinent structure of the line adapter as indicated in FIG.
1, a stop flip-flop 11 will have been set by a pulse on a Set Stop
line 12 from the TCU, see FIG. 47 of the above patent, to put a
control voltage on its upper output 13. A transmit flip-flop 14
will be set for receive by a pulse on its initiate receive line 15,
see FIG. 46 of the Richard et al. patent and can be set for
transmission by a pulse on its initiate transmit line 16, see FIG.
47 of the patent. The output line 18 of transmit flip-flop 14 will
be at a significant level when the flip-flop has been set for
receiving.
Data will be received on a line 20 which may be either a switched
private wire or, more usually, the output of a commercial data set
and will be in the form of mark and space signals. A converter 21
will change the signal levels on line 20 to voltages appropriate to
a date processing unit and will supply such voltages to its output
line 22 where a mark will be represented by a significant voltage
level. An inverter 23 is responsive to the voltage levels on line
22 and has an output on line 24 where a significant voltage level
represents a space signal.
The line adapter will be controlled by a pair of oscillators, not
shown, with the output of the fast oscillator received on line 25,
FIG. 2, and the output of the slow oscillator being received on a
line 26. The oscillator frequencies will be set at some multiple,
seven in the described embodiment, of the pulse repetition rate of
the signals to be received or transmitted. Each line 25, 26 is an
input to an AND circuit 27, 28 respectively whose outputs are
combined in an OR circuit 29. A flip-flop 30 has its outputs as the
other inputs of ANDs 27 and 28 so that depending on the setting of
flip-flop 30, one or the other of the oscillator inputs on lines 25
and 26 will be gated through the OR 29 to the oscillator line 31.
The flip-flop 30 may be set to gate the fast oscillator output on
line 25 by a pulse on a set fast oscillator line 34 or by the
initiate receive pulse on line 15, the two inputs being combined in
OR 35 on the set input of flip-flop 30. The flip-flop 30 may be set
to pass the slow oscillator signal on line 26 by a pulse on the set
slow oscillator line 36 or by a pulse on a line 37 from an AND
circuit 38 to be described at a later point. The two pulse inputs
are combined in OR circuit 39 on the reset input of flip-flop 30.
The flip-flop 30 will initially be set to gate the Fast Oscillator
pulses on line 25 by the initiate receive pulse on line 15. The set
pulses on lines 34 and 36 are supplied by the connected TCU,
usually during transmit operations, at which time the TCU knows the
receiving speed of the terminal with which it will be
communicating.
When set to receive, the line adapter will be set into operation by
receipt of a space signal on line 20, FIG. 1. The antecedent mark
signal on line 22 is one input of an AND circuit 41 which has a
strobe count 0 signal on line 42 and an inverted oscillator signal
on line 43 as its other inputs. The inverted oscillator signal is
derived from the oscillator signal on line 31 by an inverter 44
connected between the two lines. The output pulses of AND 41 reset
the stop flip-flop 41 and will set a start flip-flop 45 so long as
the signal input remains a mark. As soon as the input signal on
line 20 changes to a space level, the significant voltage on space
line 24, the inverted oscillator signal on line 43, the Receive
output on line 18 of flip-flop 14 and the output on line 46 of
start flip-flop 45 are combined in AND circuit 47 on the set input
of a force five flip-flop 48 to set flip-flop 48. The set output of
flip-flop 48 on line 50 together with the Oscillator signal on line
31 pass through an AND 51 to put a force five signal on line 52 and
force a strobe counter 53 to a count of five. The strobe counter 53
comprises three binary flip-flops 54, 55 and 56, each having an
upper set input and a lower reset input together with a central
input lead shown as double to indicate that a pulse on the lead
will change the flip-flop to its opposite state. Each flip-flop has
an upper set output which is at a significant voltage when the
flip-flop is set to represent a one and a lower reset output
terminal which is at a significant voltage when the flip-flop is
reset to represent a zero.
The three flip-flops 54, 55, and 56 are connected to form a binary
counter with the reset output on line 57 of flip-flop 54 connected
to the central input of flip-flop 55 and the reset output line 59
of flip-flop 55 connected to the central input of flip-flop 56. The
counter is advanced by Oscillator pulses on line 31 which pulses
are gated through an AND circuit 61 by a circuit to be later
described, and over line 62 to the central input of flip-flop 54.
The strobe counter 53 can be forced to an initial setting of 101
representing a five by a pulse on the force five line 52 which
connects to the set input of flip-flop 56, to OR 63 on the reset
input of flip-flop 55 and to OR 64 on the set input of flip-flop
54. The counter 53 can also be forced to a setting of 001
representing a one by a pulse on a force one line 65 which connects
to the OR 64, the OR 63 and to the reset input of flip-flop 56. On
the outputs of flip-flops 54, 55, and 56, an AND circuit 68 has as
inputs the three reset outputs of the flip-flops of the counter 53
and will give a significant voltage output on line 42 when the
counter is at 000 representing a zero. A second AND circuit 69
receives the set outputs of flip-flops 54 and 56 and the reset
output of flip-flop 55 to give a significant voltage output on line
75 when the counter reads 101 representing a five and a third AND
circuit 70 receives the set inputs of all flip-flops and gives an
output on line 78 when the counter is at 111 representing seven.
The Count 0 output of AND 68 on line 42 is returned to an OR
circuit 72 which also receives the force one signal on line 65 and
the force five signal on line 52. The output of OR 72 is inverted
in an inverter 73 whose output is a second input of AND 61. This
signal will block incrementing of counter 53 whenever the counter
is at a count of zero and whenever the counter is being forced to
either a one or a five reading.
When the force five signal on line 52 sets strobe counter 53 to a
reading of five, the output line 75 of AND 70 becomes high and will
gate a pulse from the inverse oscillator line 43 through AND 76 to
the reset input of the force five flip-flop 48 to terminate the
force five signal. The force five signal on line 52 is also passed
into the reset input of start flip-flop 45 to terminate the start
signal as soon as the first start bit is detected at input terminal
20. Another setting of the start flip-flop 45 is prevented for the
remainder of the data character time by the lowering of the count
zero signal on line 42 which is an input to AND 41 which cannot
then pass an oscillator signal to the flip-flop 41.
After strobe counter 53 has been set to five, the oscillator pulses
on line 31 pass through AND 61 to advance the counter to six and
then to seven. When the count of seven is decoded by AND 70, the
count seven line 78 has a significant voltage and will gate a pulse
on inverse oscillator line 43 through AND 79 on the set input of
strobe flip-flop 80 to put a voltage on its strobe output line 81.
This strobe signal together with the receive signal on line 18 will
pass an oscillate pulse on line 31 through AND circuit 82 and to
sample line 83. The sample pulse on line 83 will be gated by the
not stop signal on the reset output line 84 of stop flip-flop 11
through an AND 85 to the force 1 line 65 to set the strobe counter
53 to a reading of one. A pulse on this line 65 notifies the TCU
that a line bit has been sampled and corresponds to a request for
bit service similar to a pulse on the set bit service line of FIG.
91 of the patent above. As soon as the strobe counter 53 has been
set to a one, the voltage on count seven output 78 drops and after
passing through an inverter 90, the resulting positive voltage is
applied to AND circuit 91 on the reset input of strobe flip-flop 80
and will gate the next inverse oscillator pulse on line 43 to reset
the strobe flip-flop 80. After strobe counter 53 has been forced to
one, the counter is incremented by the oscillator pulses through
AND 61 until it again reaches seven and causes another data sample
to be taken. The data sample is taken by a flip-flop 95 which is
set to a mark condition by the gating of a mark voltage on line 22
through AND 96 on the set input of flip-flop 95 under control of
the sample pulse from AND 82 or is alternatively reset to indicate
a space condition when the space voltage on line 24 is high to gate
the sample pulse from AND 82 through AND 97 on the reset input of
flip-flop 95. The upper output 98 of flip-flop 95 represents
received data and is transmitted to the TCU for further
processing.
If a terminal connected to data line 20 can have any one of a
plurality of different transmission rates, the line adapter of this
invention can automatically adjust itself to receive data at the
transmitting speed if restrictions are placed on the initial
characters transmitted at the slower speeds. The general
restriction is that no slower speed character in the initial
transmissions may have a bit at the mark level when it is time for
the stop bit of a high speed transmission to be received. It is
evident that this requirement can be satisfied if all characters
transmitted for adjusting the line adapter speed consist of only
space levels prior to the stop bit which is at the mark level. The
restriction can be relaxed somewhat by allowing the early part of
the transmitted characters to have any configuration so long as the
characters do not have mark levels overlapping the stop bit of a
faster transmission speed. A second restriction where more than two
transmission rates are used is that none of the initializing
character may have a transition from a mark to a space after the
time for sampling the stop bit of the character of highest
transmission rate. This will enable the next mark to space
transition to identify the next character start. As a specific
example, FIG. 3 indicates the characters transmitted by two
commercial terminals, i.e., an IBM 1050 and a model 33/35
Teletypewriter. The 1050 terminal operates at 134.5 baud and
transmits the PTTC/BCD code of seven bits, identified as B A 8 4 2
1 C between the start and stop bits where as the teletypewriter
terminal will use an 8 bit data interchange code of eight bits, 1 2
3 4 5 6 7 8 between the start and stop signals. At a 134.5 baud
rate, a bit time for the IBM 1050 terminal is about 7.4 ms per bit
or a total of 63.2 ms from the transition to a space level until
the sampling of the stop bit mark level. A TTY 33/35 character
could, with about a 35 percent bit time allowance for line
distortion as indicated by the lined part of the timing diagram, be
in either the sixth or seventh bit at this time and therefor the
initial characters of the TTY 33/35 must be at a space level for
these bits. This will normally be the situation where the first
group of signals sent by a TTY 33/35 is its terminal identification
which starts with a carriage return code, i.e., start 10110001
stop. This code is at the required space level for the sixth and
seventh bits. Other systems may require special characters for the
start of transmission.
The switching of the line adapter to a slower speed is controlled
by an initialize flip-flop 100, FIG. 2. This flip-flop 100 will be
set by the initiate receive pulse on line 15. When the set stop
signal is received on line 12 to indicate that the next sampled
signal will be the stop bit if the character is transmitted at the
highest transmission rate, the status of the data line 20 is
tested. If the line 20 is at the mark level and the stop flip-flop
11 has been set, a sample pulse on line 83 will pass through AND
101 and OR 102 on the reset input of flip-flop 100 to reset the
flip-flop. Since this mark level is the correct stop bit for the
higher oscillator rate which was assumed as being transmitted, no
other change is needed and the higher speed signal is continued on
line 31.
If, however, the data line 20 is at a space level at this time, it
indicates that a slower speed character is being received and that
the oscillator signal on line 51 should be changed to a slower
speed. This is performed by AND 38 which receives the initialize
signal on output line 103 from the flip-flop 100, the stop signal
on line 13, the space level signal on line 24 and the sample signal
on line 83 to put a signal on line 37 to OR 39 to reset flip-flop
30. This will block AND 27 to stop the faster oscillator signal on
line 25 and condition AND 28 to pass the slower oscillator signal
on line 26. The switching of flip-flop 30 to the slower speed
oscillator, will condition AND 104 to pass an inverted oscillator
signal on line 43 to OR 102 to reset the initialize flip-flop
100.
A stop error detector flip-flop 106 is provided to put a signal on
its output line 107 if at any time after the rate of transmission
is determined, a character is not at the mark level when the stop
flip-flop 11 is set. Flip-flop 106 is reset when the line adapter
is set to receive by the initiate receive signal on line 15 or by a
special reset stop error signal from the attached TCU on a line
108, both signals being fed into an OR 109 on the reset input of
flip-flop 106. If a stop error is present at a later time, the stop
flip-flop 11 will have been set to put a signal on line 13, the
initialize flip-flop 100 will have been reset to put a signal on
its reset output line 110 and if the data line 24 is at a space
level when the sample signal appears on line 83 the stop error
flip-flop 106 will be set by the sample signal through AND 111 to
indicate to the attached TCU that an incorrect character format has
been received, see FIG. 56 of the above Richard et al. patent.
Two further embodiments are shown diagrammatically in FIGS. 4 and 5
to enable reception by the line adapter of messages of more than
two different transmission rates. The embodiment of FIG. 4 shifts
to the next lower oscillator speed each time a space level is
detected at the time the transmission would be at a mark level if
the connected terminal were transmitting at the speed for the
oscillator which is then connected. As shown in FIG. 4, a type of
shift register is utilized for connecting one of a plurality of
oscillators to the line adapter oscillator line 13. Each oscillator
output is received on a line 125A, 125B--125N and is gated through
an AND 127A, 127B--127N to OR 129 whose output is line 13. Each AND
gate 127 is gated to pass its oscillator signal when a
corresponding flip-flop 130A, 130B--130N is set. The initiate
receive signal on line 15 is used to control the flip-flop 130A by
being passed through OR 135A on its set input to flip-flop 130A to
gate the highest oscillator signal and to reset all other flip-flop
130B--130N by being passed through the OR's 139B--139N on their
reset inputs. The same signal will also set the initiate flip-flop
100. Each flip-flop 130 can be set to gate its associated
oscillator output on line 125 through AND 127 to line 13 by a
signal applied on a line 134 to the OR 135 on its set input and can
be reset to block such AND gate 127 by a signal on a line 136 to
the OR 139 and its reset input. The signals on lines 134 and 136
will normally be controlled by the associated TCU for selecting a
proper oscillator for transmission purposes, it being assumed that
the TCU will be aware of the data speed of the terminal to which a
message is being sent.
The flip-flops 130 are connected as a shift register by a
connection from the set output of each flip-flop 130 to the OR 139
on the next earlier flip-flop of the register and to an AND 140
whose output goes to the OR 135 on the set input of the next later
flip-flop. The AND's 140 also receive inputs from lines 83, 24, 13,
and 103 so that each time the TCU sets the stop flip-flop 11 and
the data line 24 is at a space level when sampled by the signal on
line 83, the register will shift to set the next flip-flop 130 for
the next slower transmission speed. It will be understood that the
shift register will also include such other connections between the
flip-flops 130 as are conventionally provided to prevent race
conditions. The AND circuit 204 has its output fed back into the OR
102 on the reset input of flip-flop 100 to reset the initialize
flip-flop when the flip-flop 130N is set for reception of the
slowest speed of transmission. For any other speed, the initialize
flip-flop 100 will be reset by the sampling of a stop bit when the
flip-flops 130 are set to receive data characters at the
transmission speed of the connected terminal.
Another embodiment is shown in FIG. 5 to make a selection of the
proper receiving speed on the first transmitted character. In this
embodiment, a counter is driven by the highest speed oscillator and
has its outputs decoded to successively test the data lines to
detect the first mark level at a possible stop time. In this FIG.
as in FIG. 4, each oscillator output for a possible transmission
speed is received on a line 125A--125N and is passed through a gate
127A--127N to an OR 129 and to oscillator line 31. Each gate 127 is
controlled by a flip-flop 130A--130N with only one flip-flop 130
being set at any time to gate only one oscillator output to line
31. Each flip-flop 130 has an OR 135 circuit on its set input and
an OR circuit 139 on its reset input with an input 134 136
respectively, going to the connected processor so that the line
adapter can be set to any transmission speed under central
processor control. Normally, the flip-flops 130 will be initially
conditioned so that flip-flop 130A is set and the remaining
flip-flops 130B--130N are reset so that the highest frequency
oscillator output on line 125A is gated to line 31. This initial
setting is done by the initiate receive pulse on line 15 which is
passed through OR 135A and OR's 139B--139N.
To select the correct oscillator output for the signals being
received, a counter 150 is provided to count the oscillator signals
on line 31 for a period equal to at least the length of the slowest
character to be received. A connection from line 15 to counter 150
is provided to set the counter to a reading of zero whenever the
initiate receive line 15 is pulsed to set the line adapter to a
starting condition. The counter is incremented by oscillator
signals on line 31 passing through AND 151 which receives the
signal on line 103 from initiate flip-flop 100, the not-stop signal
on line 84 from flip-flop 11 and the output from an OR circuit 152.
OR 152 receives the Force 5 signal on line 52 from flip-flop 48 and
a not-zero signal on a line 153 from a decoder 154 on the outputs
of counter 151. Normally both inputs to OR 151 are low but when a
space signal is received on line 24, the line 52 becomes high to
pass the first oscillator pulse into counter 150 after which line
153 becomes high to maintain AND 151 gated to continue
counting.
Decoder 154 has a number of outputs 155, one for the stop bit in
each possible received character and puts a signal on the
associated output at the time a possible stop bit can be received.
If the character being received is at the highest transmission
rate, output line 155A is up when the stop bit for this character
is received and the mark level on line 22 together with the signal
on line 155A will pass through AND 101 and OR 102 to reset
initialize flip-flop 100 and drop the signal on line 103 to stop
the counter 150. If the line 22 is at a space level, the counter
continues to count until output 155B receives a signal. At this
time AND 156B will be gated by a mark level on line 22 and the
initialize signal on line 103 to set flip-flop 103B and gate out
the second fastest oscillator signal on line 125B to line 31 if a
mark level was detected. Setting of flip-flop 130B will put its
output signal through OR 139A to reset flip-flop 130A and will send
a signal through OR 157 to reset flip-flop 100. If the mark level
is not detected at this time, the other output lines 155 receive
signals in turn to set their associated flip-flop 130 when a mark
level is detected. Each flip-flop 130 when set will reset flip-flop
130A through OR 139A and will reset flip-flop 100 through OR 157
and will also notify the central processor what data receiving
speed has been selected. The processor will then be enabled to set
the stop flip-flop 11 at the appropriate times. If an erroneous
condition has occurred on the transmission lines and the counter
150 counts to a reading higher than any possible interval for any
connected terminal, an error line 157 will receive a signal to
notify the processor that an error has occurred and the line
adapter needs to be reset.
It will be thus apparent that the line adapters disclosed will
automatically select the correct speed for reception from terminals
of either of two different speed terminals or for systems having
terminals of more than two different speeds connectable to a line
adapter.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood
by those skilled in the art that the foregoing and other changes in
form and details may be made therein without departing from the
spirit and scope of the invention.
* * * * *