Programming Considerations
Section Five
GPIB-1014P User Manual
5-10
© National Instruments Corporation
Parallel Polls
Parallel Polls are used by the GPIB Active Controller to check the status of several devices
simultaneously. The meaning of the status returned by the devices being polled is device-
dependent. There are two general ways in which Parallel Polls are useful:
•
When the GPIB Controller sees SRQ asserted in a system with several devices, it can quickly
determine which one needs to be serially polled usually using only one Parallel Poll.
•
In systems in which the Controller response time requirement to service a device is low and the
number of devices is small, Parallel Polls can replace Serial Polls entirely, provided that the
Controller polls frequently.
Although the Controller can obtain a Parallel Poll response quickly and at any time, there can be
considerable front-end overhead during initialization to configure the devices to respond
appropriately. This is contrasted with Serial Polls, where the overhead, in the form of addressing
and enabling command messages, occurs with each poll.
Conducting a Parallel Poll
The TLC as Active Controller has the capability to conduct a Parallel Poll. When the Execute
Parallel Poll auxiliary command is issued and the TLC internal local message rpp is set, the Parallel
Poll is executed (that is, the GPIB message IDY is sent true) as soon as the TLC Controller
interface function is placed in the proper state (CAWS or CACS). The Parallel Poll Response
(PPR) is automatically read from the GPIB DIO lines into the CPTR and the rpp local message is
cleared. A program can determine that the Parallel Poll operation is complete based on the condition
of CO (CO=1 when the poll is complete). The response can be obtained by reading the contents of
the CPTR. The response is held in the CPTR until a GPIB command is transmitted or the TLC
Controller function becomes inactive.
In response to IDY, each device participating in the Parallel Poll drives one and only one GPIB DIO
line (its Parallel Poll response or PPRn) active true or passive false, while it drives the other lines
passive false.
Since there are eight data lines, and for each line there can be one response (true or false) for each
device (2 lines/device), there are 16 possible responses. The line that a device uses and how that
device drives the line depends on how it was configured and whether its local individual status
message (ist) is one or zero. Thus, each device on the GPIB can be configured to drive its assigned
DIO line true if ist=1 and to drive the DIO line false if ist=0; or it can be configured to do exactly
the opposite; that is, to drive the DIO line true if ist=0 and false if ist=1. (The meaning of the value
of ist, whether one or zero, is system-dependent or device-dependent.)
Because the data lines are driven Open Collector during Parallel Polls, more than one device can
respond on each line. The device or devices asserting the line true overrides any device asserting the
line false. The Controller must know in advance whether a true response means the local ist
message of the device is one or zero. To do this, the device must be configured to respond in the
desired way. Two methods can be used to accomplish this:
•
Local configuration (Parallel Poll function subset PP2) involves assigning a response line
and sense from the device side in a manner similar to assigning the device GPIB address. Thus,
one device might be assigned to respond with remote message PPR1 (driving DIO1), while a
