Asynchronous Terminals and Printer Processes Programming Manual
PRINTER PROGRAMMING CONSIDERATIONS
Reverse Channel (RC)
RC supports printers whose pin-12 polarity definition is the
opposite of those that use TPAUSE. The polarity of the incoming
RC signal is defined as follows:
RC not busy = RC signal in the on = send data
or logic 1 state
RC busy = RC signal in the off = do not
or logic 0 state send data
RC busy indicates that the printer's buffer has reached the high-
water mark, is out of paper, offline, or otherwise not ready.
The I/O process should not send any more data. Conversely, RC
not busy indicates that the buffer has reached the low-water
mark, and the I/O process can once again send data.
Upon detecting RC busy, the I/O process halts data transmission
at the end of the current transmission block and activates
a delay timer. A timeout occurs, and the I/O operation is
terminated if the amount of time specified by the DELAYTIME
SYSGEN modifier elapses before RC again becomes not busy.
To select reverse channel sensing, specify the RC, XFERSIZE, and
DELAYTIME SYSGEN modifiers.
DELAYTIME SYSGEN Modifier
The DELAYTIME
n
modifier provides a specified amount of time for
the printer's buffer to reach its low-water mark after the data
flow is halted. Note that DELAYTIME applies to DTR, TPAUSE,
REVTPAUSE, and RC.
The I/O process activates the delay timer whenever the printer
halts the data flow. A halt in the data flow is caused by
the received data filling the buffer past the high-water mark
or by error conditions, such as an operator-correctable fault or
printer offline condition.
The value
n
, in 10-millisecond units, must be in the range 0
through 32767. The default delay time is 12000 (two minutes).
Use the following formula to determine a reasonable DELAYTIME
based on the printer's buffer size, low-water mark, and operating
speed:
DELAYTIME = [[buffer size - low water mark]/speed] * 100 * 2]
The buffer size, low-water mark, and high-water mark (all
expressed in number of characters) should be available from the
printer vendor's specifications.
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