November 1985
National Instruments
controller configures the card reader (at address 6) to
respond positively on DIO line 4 (which sets bit 3 of the response
byte) when free by sending the configuration byte 01100011. (The
S bit is set to 0, the value of bits O-2 is 3.)
PPC
When a parallel poll is conducted one of two things will happen.
If the card reader is free, bit 3 of the response byte will be 1; if it is
busy, bit 3 will be 0. When the device is free its ist bit is 0 and
because this equals the value of the bit, the device asserts DIO
line 4.
The active controller configures the line printers to all respond
positively on DIO line 1 when busy. In this case, the
ppr,s
argument for each of them is
Thus, the configuration byte for
each of them is 01101000 (hex 68). When a parallel poll is
conducted,
controller can immediately
out if all line printers
are free because the response in this situation will be 0. If any line
printer is busy, bit 0 of the parallel poll response will be 1, corres-
ponding to DIO line 1 being asserted. But what if the active con-
troller wants to
if one line printer is free? If the controller
reconfigures the line printers to respond positively when free
=
configuration byte = 01
then if any
device is free, it will drive the DIO line to 1.
controller can
use
bit correspondence for different types of information.
120
APPENDIX F PARALLEL POLLING