Parallel Polling
Appendix E
GPIB-SCSI-A User Manual
E-6
© National Instruments Corp.
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
7
6 5 4 3 2 1
0
Byte
0
CDH
1
6
0
2
0
0
0
3
0
| 0
4
63H
5
0
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.
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Страница 355: ...Appendix C Operation of the GPIB National Instruments Corp C 7 GPIB SCSI A User Manual Figure C 2 Linear Configuration ...
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