Chapter 7
NI-488.2 Programming Techniques
NI-488.2 User Manual for Windows
7-12
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the application could monitor the status bits TACS (Talker Active State)
and LACS (Listener Active State) to determine when to send data to or
receive data from the Controller. The application could also monitor the
DCAS (Device Clear Active State) and DTAS (Device Trigger Active
State) bits to determine if the Controller has sent the device clear (DCL or
SDC) or trigger (GET) messages to the interface. If the application detects
a device clear from the Controller, it might reset the internal state of
message buffers. If it detects a trigger message from the Controller, the
application might begin an operation, such as taking a voltage reading if the
application is actually acting as a voltmeter.
Serial Polling
You can use serial polling to obtain specific information from GPIB
devices when they request service. When the GPIB SRQ line is asserted, it
signals the Controller that a service request is pending. The Controller must
then determine which device asserted the SRQ line and respond
accordingly. The most common method for SRQ detection and servicing is
the serial poll. This section describes how to set up your application to
detect and respond to service requests from GPIB devices.
Service Requests from IEEE 488 Devices
IEEE 488 devices request service from the GPIB Controller by asserting
the GPIB SRQ line. When the Controller acknowledges the SRQ, it serial
polls each open device on the bus to determine which device requested
service. Any device requesting service returns a status byte with bit 6 set
and then unasserts the SRQ line. Devices not requesting service return a
status byte with bit 6 cleared. Manufacturers of IEEE 488 devices use lower
order bits to communicate the reason for the service request or to
summarize the state of the device.
Service Requests from IEEE 488.2 Devices
The IEEE 488.2 standard refined the bit assignments in the status byte. In
addition to setting bit 6 when requesting service, IEEE 488.2 devices also
use two other bits to specify their status. Bit 4, the Message Available bit
(MAV), is set when the device is ready to send previously queried data.
Bit 5, the Event Status bit (ESB), is set if one or more of the enabled
IEEE 488.2 events occurs. These events include power-on, user request,
command error, execution error, device dependent error, query error,
request control, and operation complete. The device can assert SRQ when
ESB or MAV are set, or when a manufacturer-defined condition occurs.
