National Instruments GPIB-SCSI-A User Manual page 374

Parallel Polling
The controller configures the scanner (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.)
Bit
Byte
0
1
2
3
4
5
When a parallel poll is conducted, one of two things will happen. If the
scanner is free, bit 3 of the response byte will be 1; if it is busy, bit 3 will be
0. When the scanner is free, its ist bit is 0 and because this equals the value
of the S bit, the device asserts DIO line 4.
The controller configures all of the line printers to respond positively on
DIO line 1 when busy. In this case, 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 bit 0 of the
parallel poll response 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 controller wants to know if one line printer is free? If
the controller reconfigures the line printers to respond positively when free
(configuration byte = 011000000), 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.
GPIB-SCSI-A User Manual
7 6 5
6
0
4 3 2
CD H
0
63 H
0
E-6 © National Instruments Corp.
Appendix E
1 0
0
0 0
| 0