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DIDS-402-2AM13
4-6.1 Request-to-Send
The purpose of the request-to-send line (circuit CA) is to notify the con-
nected interface device that the Display Terminal has a message available for
transmission.
This line couples a control condition of ON (-6v ::I:lv) to the con-
nected device after the END and XMIT keys are depressed (two-step transmit) or
when only the END key is depressed (one-step transmit option).
The request-to-
send line remains ON until the last data character (ETX) has been transrrlitted
from the Display Terminal.
4-6.2 Clear-to-Send
The purpose of the c1ear-to-send line (circuit CB) is to notify the Display
Terminal that communication with the CPU has been achieved and the message
available for transfer may be transmitted.
The c1ear-to- send line as SUrrles a
control condition of ON at the direction of the connected CPU I/O controller and
remains ON for as long as the request-to-send line is ON.
4-6.3 Transmitted Data
The transmitted data line (circuit BA) is used to couple serial bit data at
1200 baud from the Display Terminal to the CPU I/O controller.
When the
Display Terminal is not actively transmitting data (such as between characters).
this line is held at a marking condition (+6v ::I:lv).
4-6.4 Received Data
The received data line (circuit BB) is used to couple serial bit data at
1200 baud from the CPU I/O controller to the Display Terminal.
The received
data line is enabled at all times other than when the Display Terminal is trans-
mitting data.
4-6.5 Ground
. The signal ground line establishes a common ground reference for all
CPU I/O controller to display terminal interface line s.
4-6.6 Transmit/Receive Conditions
In
order for the display terminal to transmit or receive data. the following
conditions must be true:
To Transmit
To Receive
Request-to-Send
ON
OFF
Cle~r-to-Send
ON
OFF
4-7 CONVERSATIONAL MODES
The Display Terminal communicates with the CPU in one of two conversa-
tional modes: enquiry-response or polling.
Each mode has advantages and
disadvantages which must be considered during system planning to arrive at
the mode most suitable for the particular system.
An advantage of the enquiry-
4-6
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