Parallel Polling
Appendix E
GPIB-SCSI-A User Manual
E-4
© National Instruments Corp.
The value of this byte is hex 6A where:
U = 0
Enable Parallel Poll responses
S = 1
When ist = 1 the device will assert DIO
line 3 (which corresponds to 010 in bits 0 through 2) in
response to a Parallel Poll.
The ppc function sends the talk address of the GPIB-SCSI-A, unlisten, the
listen address of the device, Parallel Poll Configure, then the parallel poll
message byte (hex 6A in the above example). The parallel poll message
byte can be any valid PPE or PPD message.
In order for the ppc function to have an effect, the device that it is
configuring must be using PP subset PP1. If the device is not using PP
subset PP1, then it does not allow itself to be configured by an external
controller, and will probably ignore the configuration.
Use the ppu function to unconfigure all devices on the GPIB from
responding to parallel polls. This function causes the GPIB Parallel Poll
Unconfigure (PPU) message to be sent. This message has the same effect
as using the ppc function to send PPD messages to every device on the
GPIB. Like the ppc function, ppu will only affect devices that are using PP
subset PP1.
Issuing Local Configurations in S Mode
In S Mode, the ppc function can also be used to locally configure the GPIB-
SCSI-A itself. Local configuration is indicated by setting the Me bit in the
Command Descriptor Block to 1. For example, if you want to configure the
GPIB-SCSI-A to respond on DIO line 5 when its ist bit is 0, the Me bit
should be set to 1, and the parallel poll message should be set to hex 64
(binary 0110 0100).
When configuring the GPIB-SCSI-A itself, the ppc function does not send
messages out on the GPIB. Instead, it internally configures itself to respond
as indicated by the parallel poll message.
In order for the ppc function to have an effect, the GPIB-SCSI-A must be
using PP subset PP2. If the GPIB-SCSI-A is not configured to use PP
subset PP2, then it cannot allow itself to be locally configured, and will
return an ECAP error. The GPIB-SCSI-A can be configured to use PP
subset PP2 with the config function.
