Optional GPIB (IEEE 488) Interface (Model Z9216-4 Only)
The IEEE-488 Interface Standard
The General Purpose Interface Bus (GPIB), also known as the IEEE-488 interface, is an 8-bit parallel
bus common on test equipment. The IEEE-488 standard was proposed by Hewlett-Packard in the
late 1970s and has undergone several revisions. HP documentation (including data sheets and
manuals) calls it HP-IB, or Hewlett-Packard Interface Bus. It allows up to 15 intelligent devices to
share a single bus, with the slowest device participating in the control and data transfer handshakes to
drive the speed of the transaction. The maximum data rate is about one megabit per second.
In June 1987, the IEEE approved a new standard for programmable instruments called ANSI/IEEE
Std. 488.2-1987 Codes, Formats, Protocols, and Common Commands. It is backward compatible
with the IEEE Standard Digital Interface for Programmable Instrumentation, ANSI/IEEE 488-1978
(now 488.1). HP-IB is Hewlett-Packard's implementation of IEEE 488.1. The Standard Commands
for Programmable Instruments (SCPI) portion of IEEE 488.2 uses the command structures defined in
IEEE 488.2 to create a single, comprehensive programming command set for use with any SCPI
instrument.
GPIB devices communicate with other GPIB devices by sending device-dependent messages and
interface messages on the bus. The devices can be Talkers, Listeners, 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. A digital voltmeter, for example,
is a Talker and also a Listener.
The GPIB is similar to a computer bus, but instead of connecting different PC cards on a
motherboard, the GPIB connects them by standard cables. The role of the GPIB Controller is
comparable to that of a computer’s CPU or, more aptly, that of switching center of a telephone
system. The switching center (Controller) monitors the communications network (GPIB). When the
Controller notices that a party (device) wants to make a call (send a data message), it connects the
caller (Talker) to the receiver (Listener). The Controller usually addresses (enables) a Talker and a
Listener before the data message can be sent.
Some GPIB configurations do not require a Controller, such as when a device that is always a Talker
is connected to one or more listen-only devices. A Controller is necessary when the active Talker or
Listener must be changed. The Controller function is usually handled by a computer, which with the
appropriate software and hardware can perform the roles of Controller and Talker/Listener.
As detailed in Table 3-3, the GPIB interface system utilizes a 24-pin ribbon-type connector, with
16 signal lines and eight ground-return or shield-drain lines. The 16 signal lines within the passive
interconnecting HP-IB (IEEE-488) cable are grouped into three clusters according to their functions:
Data Bus (eight lines): DI01 to DI08, data lines that carry either data or command messages. All commands
and most data use the 7-bit ASCII code set, the eighth bit, DI08, being used for parity when parity is used.
Handshaking or Data Byte Transfer Control Bus (three lines): NRFD (not ready for data), NDAC (not data
accepted), and DAV (data valid). The use of these lines, together with the settling time properties of the line
and its termination, determine the speed of the data transfer.
General Interface Management Bus (five lines): ATN (attention), IFC (interface clear), REN (remote enable),
SRQ (service request), and EOI (end or identify).
2
To Next Page
Loading...
