Specifications

University of Hertfordshire

quickly gained popularity, because of its high transfer data rate (nominally 1 bytes/s). In 1975, it

was later accepted as IEEE Standard 488-1975, and in 1987 it has evolved to ANSI/IEEE Standard

488.1-1987. ANSI/IEEE 488.2-1987 enhanced and strengthened 488.1-1987 by defining precisely

how controllers and instruments communicate. In addition, Standard Commands for Programmable

Instruments (SCPI) took the command structures defined in IEEE 488.2 and created a single,

comprehensive programming command set that is used with any SCPI instrument. Today, the name

General Purpose Interface Bus (GPIB) is more widely used than HP-IB.

4.2.2 Types of GPIB Messages

There are two types of GPIB Message, device-dependent messages, also called data or data

messages, are used when devices communicating with other GPIB devices and interface messages,

which contain device specific information, such as programming instructions, measurement results,

machine status, and data files and Interface-message, used to through the interface system, interface

messages manage the bus and can perform functions like initializing the bus, addressing and

unaddressing devices, and setting remote or local programming device modes. Interface messages

also called commands or commands messages.

4.2.3 Talker, Listeners and Controllers

GPIB Devices can be Talkers, Listeners, and/or Controllers. A Talker sends data messages to one or

more Listeners, which receive the data. The Controller manages the flow of information on the

GPIB by sending commands to all devices. The Spectrum Analyser, in this project, is a Talker and is

also a Listener.

4.2.4 IEEE 488 Shake Hands

The standard IEEE 488.1 3-wire-handshake (shown in Figure 4.2) requires the Listener to unassert

Not Ready for Data (NRFD), the Talker to assert the Data Valid (DAV) signal to indicate to the

Listener that a data byte is available, and for the Listener to unassert the Not Data Accepted (NDAC)

signal when it has accepted that byte. A byte cannot transfer in less than the time it takes for the

following events to occur – NRFD to propagate to the Talker, DAV signal to propagate to all

Listeners, the Listeners to accept the byte and assert NDAC, the NDAC signal to propagate back to

the Talker, and the Talker to allow a settling time (T1) before asserting DAV again.