Data Acquisition

Figure 8.8IEEE 488.2 status reporting modelThis model builds upon the status byte definition of IEEE 488.1, which only had bit 6defined as the RQS (request service) bit. The RQS bit is set to indicate that a device isrequesting service by asserting the service request (SRQ) line. A further two bits aredefined, the event status bit ESB) and the message available (MAV) bit. Other bits aredefined by the manufacturer. The ESB indicates that one of the standard events, asdefined in the standard event status register, has occurred. Only the events for which theuser has set the corresponding bit in the standard event status register will cause the ESBto be set in the status byte. The MAV bit is used to indicate if a message is available inthe device’s output queue.Whether a device asserts the service request (SRQ) line when one of the bits in itsstatus register becomes set, is enabled or disabled, using the service request enableregister. If a bit is set by the user in this register, then the setting of the corresponding bitin the status register will cause the device to assert the SRQ line.IEEE 488.2 also designates a minimum set of standard commands that a device musthave. These are not new bus command messages, but are new data messages common toall devices. Table 8.4 lists the mandatory IEEE 488.2 commands, indicating the commandgroup to which each command belongs.