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525B
Users Manual
4-10
IEEE-488 Interface Overview
The IEEE-488 parallel interface sends commands as data and receives
measurements and messages as data. The maximum data exchange rate is 1 Mbyte,
with a maximum distance of 20 meters for the sum length of the connecting cables.
A single cable should not exceed 4 meters in length. Some commands are reserved
for RS-232 serial operation because these functions must be implemented as IEEE
messages per the IEEE Standards. For example, the command REMOTE could be
sent as data over the IEEE-488 interface to place the Calibrator into remote, but it
is not because the IEEE Standards call for the remote function to be sent to the
device as the uniline message REN. This is also true for several other commands
and functions, as shown below, with their equivalent RS-232 emulation. A
summary of IEEE-488 messages is shown in Table 4-3.
Table 4-3. RS-232 Emulation of IEEE-488 Messages
IEEE-488 Message RS-232 Equivalent
GTL LOCAL command
GTR REMOTE command
LLO LOCKOUT command
SDC, DCL (not emulated on RS-232)
GET (not emulated on RS-232)
SPE, SPD (not emulated on RS-232)
UNL, UNT (not emulated on RS-232)
The IEEE-488 interface is based on the IEEE Standards 488.1 and 488.2. For
detailed information, refer to the IEEE-488.1 and IEEE-488.2 standards.
IEEE-488.1 IEEE-488.1 is the hardware portion of the interface. The parallel
signal lines are divided into eight lines for the data bus, three lines for the
handshake, and five lines for bus management. The handshake lines take care of
the timing for data exchange. The bus management lines control the operation of
data exchange. The ATN line indicates the use of the DIO lines for addresses or
messages (true), or for DIO data (false). The EOI line is used with the data lines to
mark the end of a message, and with the ATN line for polling. The SRQ line is
used by the devices to indicate to the controller that they require service. The IFC
line is used by the controller to quickly get all the devices on the bus to stop
talking and start listening. The REN line is used to implement the remote/local
states.
IEEE-488.2 IEEE-488.2 is the software portion of the interface, specifying data
formats, common commands, message exchange protocol and the status register
implementation. Use the following to decode the columns in Table 4-4.
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