Parallel Polling
Appendix F
GPIB-232CT User Manual
F-4
© National Instruments Corp.
The Active 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 0 through 2 is 3.)
PPC
6,4,0
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 all of the line printers to respond
positively on DIO line 1 when busy. In this case, the
ppr,s
argument for
each of them is
0,1
. 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, corresponding to DIO line 1 being asserted. However,
what if the Active Controller wants to know if one line printer is free? If
the Controller reconfigures the line printers to respond positively when free
(dio, s = 0, 0; configuration byte = 01100000), then if any device is free, it
will drive the DIO line to 1. Thus, the Controller can use S-bit/IST bit
correspondence for different types of information.
