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DIDS-402-2AM13
c.
The stop bit, b9' is always a MARK (+6v ±lv) and is always the last
bit of a character.
The stop bit is used to inform either the CPU or
the Display Terminal that one complete character has been received.
4-10 MESSAGE FORMAT
The general message format for all data communication between the
Display Terminal and the CPU is: STX, DA, TEXT or COMMAND, and ETX.
These terms are described below.
STX
DA
TEXT
or
COMMAND -
ETX
The start-of-text character is always the first character
of any data me ssage transferred between the Display
Terminal and the CPU.
In the transmit mode (Display
Terminal is issuing an enquiry or responding to a read
directive), the STX character is jammed into buffer
re gister D3 during the initial transmit sequence.
In
the
receive mode, the STX character is decoded and if it is
immediately followed by the correct device address, the
Display Terminal is readied to receive the remainder of
the data message.
The device address character is a 7-bit code which the
Display Terminal samples for its binary value (receive)
or jams into buffer register D2 (transmit).
In
the
receive mode, the DA is examined in buffer register Dl
to determine whether the incoming message is intended
for display terminal use.
In the transmit mode, the DA
is jammed into the buffer register and transmitted to
inform the CPU of the message origin. ,In the polling
mode, the DA is also used to indicate a read or write
directive.
If the incoming message contains a 1 in the
MSB position of the DA, the Display Terminal responds
with an available message or with a Ina-business I response.
If the incoming me s'sage contains a 0 in the MSB position of
the DA, the Display Terminal writes data characters
received after the DA onto the CRT screen.
This portion of the message contains up to 1039 data
characters which may be in the form of displayable
or nondisplayable characters.
Normally, command
codes are not displayable and either position the cur-
sor or edit and format the message.
The end-of-text character is normally the last character
of any transmitted message.
In
the transmit mode, the
detection of an ETX character terminates the transmit
mode.
In
the receive mode, ETX is used to determine
the end of the message arriving from the CPU.
In
the
polling mode, multiterminal sites may be optionally con-
figured so that the last Display Terminal to transmit
sub.stitutes an EOT code for ETX.
This is the only
instance where ETX does not signify the end of a particu-
lar message.
4-11
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