Bus Remote Interface Gpib / Rs-232 (Option); Gpib Messages; Functions; Signals And Lines - Extech Instruments 7470 Operation Manual

9. Bus Remote Interface GPIB / RS-232 (Option)

This section provides information on the proper use and configuration of bus remote
interface. The RS-232 interface also uses the same command set as the GPIB
interface for setting of test parameters. However, many functions of the GPIB 488.2
interface are not available through RS-232. The IEEE-488 interface included with
High Withstand Voltage Tester conforms to the requirements of the IEEE-488.2
standard.

9.1. GPIB Messages

There are typically two types of messages that GPIB devices use to communicate with
other interconnected GPIB devices;
Interface messages: Often called commands or command messages and Device
dependent messages often called data or data messages.
Data Messages: Contain information such as programming instructions or
measurement results. Command Messages perform functions such as initializing the
bus and addressing and unaddressing devices.

9.2. Functions

A GPIB device can be a Listener, Talker, and/or Controller. A Talker sends data
messages to one or more Listeners, which receive data. A Controller manages the
information flow on the GPIB by sending commands to all devices. The GPIB bus is
much like a computer bus except a computer has circuit cards connected via a
backplane and the GPIB has stand-alone devices connected via a cable.

9.3. Signals and Lines

The GPIB consists of 16 signal lines and 8 ground-return or shield drain lines. The 16
signal lines are grouped into 8 data lines, 3 handshake lines, and 5 interface
management lines.
Data Lines: The eight data lines, DI01 through DI08 carry data and command
messages. The 7-bit ASCII or ISO code set is used and the eighth bit DI08 is unused.
Handshake Lines: The transfer of message bytes between devices is done via three
asynchronous control lines. Referred to as three-wire interlocked handshake. This
guarantees that message bytes on the data lines are sent and received without
transmission error.
NRFD (not ready for data) indicates when a device is ready or not ready to receive a
message byte.
NDAC (not data accepted) indicates when a device has or has not accepted a
message byte.
DAV (data valid) tells when the signals on the data lines are stable (valid) and can be
accepted safely by devices.
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