IBM 1 Series - Read Control and Write Control

Read Control and Write Control
‘Read control’ and ‘write control’ are two general purpose outputs 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 Bits 4—7
GO01 000X
Bit 7 of a Read command controls the ‘read control’ output. If a Read
command is issued with bit 7=1, the ‘read control’ output closes and
remains closed until either (1) a Read command is issued with bit 7=0 or
(2) the attachment is reset.
If a Read command is issued with bit 7=0, the ‘read control’ output opens
and remains open until a Read command is issued with bit 7=1.
Any system reset or device reset causes 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 only if
the Operate I/O (IO) condition code response to the command is
condition code 7 (command accepted).
A Read command cannot change the state of the ‘read control’ output if
the IO condition code response to the command is condition code 0
(device not attached), 1 (read busy), or 3 (command reject). The
teletypewriter attachment does not use condition code 2, 4, or 6; [O
condition code 5 is not used on a Read command.
Read Control Timing. A system reset or device reset can cause the ‘read
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 normally switches
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 through one or more
overrun receive operations until a Read command is executed.
4-20 GA34-0033