Manual

Rev. 3

Remote Interface Reference

IEEE-488 Communication Option

4.2 IEEE-488 Communication Option

The IEEE-488 communication option provides a GPIB connector on the

rear panel of the instrument for IEEE-488 (GPIB, HPIB) communications.

4.2.1 Command/return termination characters

All commands are transmitted and received as ASCII values and are case

insensitive. The Model 135/136 always transmits <LF> and EOI as the

termination for return data. The Model 135/136 can accept <CR>, <LF>,

<CR><LF>, <LF><CR>, or <LF> with EIO as termination characters from

an external IEEE-488 interface.

Only one command at a time is allowed to be transmitted by the external

IEEE-488 interface. Thus the transmission of several commands

separated by a semicolon is not allowed. The instrument uses a single 16

character buffer for input and output. Consequently, all input strings

including terminations should not be longer than 16 characters. Any

excess characters will be discarded. All alphabetical characters are case

insensitive and character encoding is in accordance with IEEE 488.2.

4.2.2 Communicating with the Model 135/136

The use of a single buffer for both input and output is a result of memory

limitations in the Model 135/136. In order to keep the external IEEE-488

interface from sending successive commands faster than the Model 135/

136 can respond, the Model 135/136 uses the Serial Poll Service Request

(SRQ) to let the external computer know it has finished processing the last

command received and is ready to send a response. This is true of all

commands. Thus sending commands to the Model 135/136 using IEEE-488

protocol is a three step process: 1) send the ASCII command, 2) wait for

SRQ, and 3) get the instrument response.

Note

API’s for some manufacturer’s cards require you to use different

functions to check for SRQ and read the serial poll status (spoll)

byte. Invoking the command to read the spoll byte may be required

to actually clear the SRQ status.

A basic flow diagram for sending an ASCII command to the Model 135/136

and receiving a response is shown on the following page. It is not necessary

to wait exclusively for the SRQ status from the instrument. Other bus

commands can be processed while waiting for the SRQ status from the

instrument.