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DIDS-402-2AMI3
The following explanations are provided to enable rapid familiarization
with the se circuits as a basis for further study.
The logic diagrams which
accompany the text are simplified versions of the master drawings contained in
Chapter 7 of this manual.
Should further visual aids be required. refer to the
complete logic diagram for Communications Control Board Al2 (Raytheon
Drawing No. 425116) in Chapter 7.
4-22. I Transmit Enable
The Display Terminal is a half-duplex device designed to both transm.it and
receive digital information.
Depending upon system requirements. control over
data transfers can be vested in either the CPU (by means of the program) or the
display terminal operator.
The following text explains the operation of the trans-
mit and receive enable circuits when either the enquiry-response or polling con-
versational modes are employed.
In addition. other options which significantly
change display terminal
~peratiori
are described on a 'used-not used' comparative
basis.
4-22. 1. I Enquiry Response
The enquiry-response conversational mode is employed in systems where
the display terminal operator is generally" responsible for initiating data transfers.
After a message has been initiated by the display terminal operator (enquiry).
all further entries on the keyboard are 'locked-out' until a CPU response is
received.
The CPU may also issue unsolicited responses to the Display Terminal.
since a constant receive condition exists at times when the transmis sion circuits
are not enabled.
To transmit an enquiry. the operator types in the desired text and termin-
ates the message by depressing the END key.
This inserts an ETX code into the
delay-line memory and the code circulates as the last character of the message.
Figure 4-24 is a logic diagram of the circuits on'the Communications
Control board that enable a transmit operation.
In the circuit quiescent state, all
the control flip-flops are reset and a high is present at the output oLAND-gate
A54.8.
This output. designated as R. is the level that allows data reception when
the Display Terminal is not actively transmitting.
In enquiry-response systems where a modem or similar device is employed,
the sequence of events is:
a.
Locate ETX code in delay-line memory (ETXSR)
b.
Set the request-to-send flip-flop. disable receive logic. and clear
buffer registers D2 and D3
c.
Receive the clear-to-send control level
d.
Enter header characters STX and DA
e.
Enable transmit steering logic
4-54
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