National Instruments GPIB-MAC User Manual page 130

November 1985
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.)
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 S 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
argument for each of them is 0,l. Thus, the configuration byte for
each of them is 01101000 (hex 68). When a parallel poll is
conducted, the controller can immediately find 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 know if one line printer is free? If the controller
reconfigures the line printers to respond positively when free
device is free, it will drive the DIO line to 1. Thus&e controller can
use S-bit/ist bit correspondence for different types of information.
120 APPENDIX F - PARALLEL POLLING
PPC 6,4,0
ppr,s