Appendix H
Parallel Polling
H-6
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Example 1
Figure H-3 shows an example parallel polling situation. The configuration
and responses are described in the following paragraphs.
Figure H-3.
Parallel Polling Setup Example 1
The PC configures the scanner (at address 6) to respond positively on DIO
line 4 when free by sending the following programming message to the
GPIB-232/485CT-A:
ppc 6,4,0<CR>
This programming message causes a PPE message of hex 63 to be sent to
the scanner. When a parallel poll is conducted by calling
rpp
, one of two
things happens. If the scanner is free, it asserts DIO line 4 (
rpp
returns
hex 8); if it is busy, it unasserts DIO line 4 (
rpp
returns hex 0). When the
scanner is free, its
ist
bit is 0, and because this equals the value of the
S
bit
in the PPE message, the device asserts DIO line 4.
PO
WER
READ
Y
TA
LK
LISTEN
BU
S
Y
FULL
GPIB-232CT-A
GPIB-232/485CT-A
Controller
IBM PC
GPIB Printers
Addresses 1, 2, and 3
Respond on DIO line 5
GPIB Scanner
Address 6
Responds on DIO line 4
ist bit = 0, S bit = 0
asserts DIO line 4
ist bit = 1, S bit = 0
DIO line 4 unasserted
ist bit = 0, S bit = 1
DIO line 5 unasserted
ist bit = 1, S bit = 1
DIO line 5 asserted
If busy:
If free:
If all free:
If any busy:
