IBM Series 1 User Manual page 238

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TNL: GN34-0368 to GA34-0033-0 (February 4, 1977)
Read Control and Write Control
Read Control and Write Control are two general purpose out-
puts of the Teletypewriter Adapter. These outputs are not
used for any standard attachment of a teletypewriter I/O
device. The Read Control and Write Control outputs are
provided for whatever use the customer can find for them.
These two outputs are solid state switch outputs and have
two states, open and closed.
Read Control. A Read command has the following format:
Bits 0-3
0001
Bits 4-7
OOOX
Bit 7 of a Read command controls the Read Control out-
put. If a Read command is issued with bit 7=1, the Read
Control output will close and remain closed until either (a)
a Read command is issued with bit 7=0 or (b) the attachment
is reset.
If a Read command is issued with bit 7=0, the Read
Control output will open and remain open until a Read
command is issued with bit 7=1.
Any system reset or device reset will cause the Read
Control output to open and remain open until a Read command
is issued with bit 7=1.
A read command can change the state of the Read Control
output if the Operate I/O
(010)
condition code response to
the command is condition code 1 (read busy), 5 (interface
data check), or 7 (command accepted) .
A Read command cannot change the state of the Read
Control output if the
010
condition code response to the
command is condition code 3 (command reject) or 0 (device
not attached). The Teletypewriter attachment does not use
condition codes 2, 4, or 6.
Read Control Timing. A system reset or device reset can
cause the
Rea~Control
output to open asynchronously to the
receiving of characters.
Under normal operation, Receive interrupts are taken
after the leading edge of the STOP bit for any given
character and before the leading edge of the START bit of
the next character. The Read command is part of the Receive
interrupt sequence. Therefore, the Read Control output will
normally switch state only in the time window that starts
with the leading edge of the STOP bit of one character and
ends with the leading edge of the START bit of the next
character.
If an overrun occurs, this time window can extend
C
. "
through one or more overrun receive operations until a Read
&
command is executed.
Teletypewriter Adapter 4-16.1