Anritsu Driver User Manual

Communicating with Anritsu’s
IEEE-488.2 Non-Compliant Instruments

Application Note

IEEE-488.2 Non-Compliance

Introduction

The purpose of this application note is to recognize the difficulties associated with remotely
controlling some of Anritsu’s instruments via the GPIB bus.This note will define the exact cause
of the difficulties and then describe a way to reliably communicate with these troublesome models.

Problem Statement

GPIB Bus

Most Anritsu instruments can be remotely
controlled by a computer using the GPIB bus.
The GPIB bus has been a mainstay in the Test
& Measurement world since 1965 and was
standardized by publication of the IEEE-488
specification in 1975.There have been two
major enhancements to this standard, with the
latest being IEEE-488.2 in 1987.

The GPIB bus is used to send a series of
8-bit bytes between instruments or between
a computer and an instrument. Most often the
bytes transmitted are standard ASCII letters
and numbers. For example, if a computer wants
to set the frequency in a signal generator to
12345 Hz, the computer might send the
command "FREQ 12345". Upon receipt of this
command, the signal generator will immediately
switch to the new 12345 frequency.That
sounds simple enough, but how did the
instrument know when it had received the
entire command? Why didn't it switch to a new
frequency of 123 after receiving the characters
"FREQ 123"? How did it know to wait until the
"5" was received? This raises the whole issue
of command termination on the GPIB bus.

Command Termination

One method of signaling the end of a
command is to add one additional special
character, known as the EOS (End of
Statement) character.The most common EOS
is "new line", otherwise known as "line feed"
or ASCII 0x0A.The receiving instrument can
simply continue to accept bytes until it receives
the designated EOS character, and then act on
all of the preceding bytes.The EOS technique
is the easiest to implement by both the
transmitting and receiving devices, but it is
slightly inefficient because an extra character
must be transmitted with each command.The
EOS technique can also become a problem if
the data to be passed over the GPIB bus
includes the EOS byte.For example, reading a
long series of binary bytes from an A-to-D
converter may eventually hit upon all possible
binary values, including the special EOS value.
This would cause the transmission to be
prematurely terminated.

The alternative to using a special termination
character is to use a termination signal outside
the normal path of the data bytes.The GPIB
bus is defined as 24 parallel lines, of which 8
lines carry the data bytes. One of the remaining
16 lines is the EOI (End or Identify) line.

The EOI line is always in one of two states;
asserted (ground) or unasserted (+5 volts).
Normally EOI is unasserted. However, a GPIB
transmitter may elect to signal the end of a
command string by asserting the EOI line at
the same time as it is transmitting the last byte
of the command.The EOI termination method
is more difficult for the sender and receiver
because 9 lines must be controlled or monitored
(8 data lines plus EOI). By not sending the
EOS character, the transmissions are slightly
faster and there are no concerns about the
data bytes matching a special EOS value.

The IEEE-488.2 standard mandates that
GPIB commands must be terminated in one
of three ways:

1. Line feed as the EOS character, with no EOI.

2. EOI during the last command character
and no line feed.

3. Both line feed and EOI, meaning EOI is
asserted during the line feed.

The IEEE-488.2 standard clearly states that
a compliant device that is receiving commands
from the GPIB bus must be able to accept
commands no matter which of the three
termination methods are used. Similarly, a
compliant device transmitting commands on
the GPIB bus must use one of these three
termination methods and any one of the
three methods is equally acceptable.

A customer desiring to control one of our
instruments via the GPIB bus can use any one
of many different programming languages or
manual utilities.These tools allow the customer
to send and receive GPIB commands to a
device that is presumably compliant with the
IEEE-488.2 standard. Most, if not all, of these
tools default to use of termination method (2)
because it is the most efficient and eliminates
potential data conflicts. Some, but not all, of the
tools can be customized to allow the use of
termination methods (1) and (3).

Non-Compliance

The fundamental problem that people have
in controlling a few Anritsu instruments arises
from the fact that those instruments do not
fully comply with the IEEE-488.2 standard.
In particular, the instruments ignore the EOI
line. Consequently, they are unable to receive
a command terminated using method (2),
the most commonly used method. Such
non-compliant instruments are, however, able
to reliably receive commands using methods
(1) and (3) because in both cases a line feed
is transmitted.

A typical user will connect their computer to
an Anritsu instrument along with maybe a few
other instruments, start up a GPIB utility such
as National Instruments' MAX (Measurement
and Automation Explorer), and attempt to
communicate with the instruments. Most often
everything works exactly as expected.
Unfortunately, a few Anritsu instruments will
not work as expected.The utility will recognize
that the instrument is on the bus at a particular
address and the instrument will go into Remote
mode and accept characters. But the
instrument will never execute the command.

A non-compliant instrument will properly
receive and store away every character of the
command and will continue to do so until it
receives the terminating line feed that it is
expecting.The transmitter, on the other hand,
sends all the characters, asserting EOI on the
last character, but because EOI was ignored
by the non-compliant instrument, the instrument
ends up waiting forever for a line feed and the
customer ends up frustrated.

The typical user of our instruments
understands the GPIB bus enough to know
that ASCII characters are transmitted back
and forth, but they often don't understand the
concept of termination methods. Even if the
user does understand termination methods, the
Remote Control Manuals for the few non­compliant instruments still may not be helpful.