Remote Operation
IEEE-488 Interface Overview
5
5-23
5-22. 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 5500A 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 5-3.
Table 5-3. RS-232 Emulation of IEEE-488 Messages
IEEE-488 Message RS-232 Equivalent
GTL LOCAL command
GTR REMOTE command
LLO LOCKOUT command
SDC, DCL ^C (<Cntl> C) character [clear the device]
GET ^T (<Cntl> T) character [execute a group trigger]
SPE, SPD ^P (<Cntl> P) character [print the serial poll string]
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 standards IEEE-488.1 and IEEE-488.2.
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 Figure 5-7. Appendix D shows a typical
IEEE-488 connector and pin assignments.
Type M - Multiline
U - Uniline
Class AC - Addressed Command DD - Device Dependent
AD - Address (Talk or listen) HS - Handshake
UC - Universal Command SE - Secondary
ST - Status
Other B1, B2, etc. - Information Bits Logic Zero = 0 = False
Blanks - Doesn’t Care condition Logic One = 1 = True
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