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.

The manual for every Anritsu instrument designed in the past decade will claim that it is fully
compliant with the IEEE-488.2 standard and in most cases, that is correct. Some non-compliant
instruments will claim they are fully compliant with IEEE-488.2, but a few pages deeper in the
manual will describe how the instrument really only accepts termination methods (1) and (3).
Some non-compliant instruments will claim they are fully compliant with IEEE-488.2 and proceed
to describe how all three termination methods are accepted, but actual use of the instrument shows
that method (2) is not accepted.

The following are a few instruments currently known to be non-compliant: MF9619, MP1555,
MP1763, MP1764, MN63, and the MP1570.

The natural question of, "WHY are these instruments non-compliant?" is beyond the scope of
this paper and would be better addressed to the Anritsu divisions responsible for designing
the instruments.

Solution

While frustrating at first, these few non-compliant instruments are perfectly reliable and will
flawlessly receive, execute, and respond to GPIB commands as long as the commands are
terminated with a line feed.The manner in which you append the line feed depends on the tool
you are using for communication.

NI-488.2 Communicator

National Instruments is the largest manufacturer of GPIB cards in the US.Every card comes with
a software utility called MAX (Measurement & Automation Explorer) as shown in Figure 1.

(Figure 1)

The upper left corner of MAX has a button
labeled "Communicate with Instrument".
When this button is clicked, the "NI-488.2
Communicator" application pops up as shown
in Figure 2.This is the utility our customers
most commonly use to manually control our
instruments. For any device that fully complies
with IEEE-488.2, this is a very easy utility to
send and receive GPIB commands.

The termination method employed by NI-488.2
Communicator is EOI-only, method (2), thus
Communicator will not communicate with our
few non-compliant instruments.Fur thermore,
there is no way to force Communicator to send
a line feed.

(In January 2001, National Instruments
acknowledged that this was a bug and
promised that some later version of
Communicator would allow line feeds.)

Fortunately, MAX includes another GPIB
tool which can send line feeds.

VISAic

VISA (Virtual Instrumentation Systems
Architecture) is a driver software architecture
developed by National Instruments to unify
instrumentation software.VISAic (VISA
Interactive Control) is a predecessor to
NI-488.2 Communicator and can be invoked
from MAX as shown in Figure 3.

The first screen to appear when VISAic is
invoked will be similar to Figure 4.VISAic
automatically scans your entire computer for all
forms of I/O, typically finding a couple RS-232
ports, a printer por t, and any instruments that
may be connected to your GPIB bus. In the
following example, one GPIB device is found
and that device has an address of 13.

(Figure 2)

(Figure 4)

(Figure 3)

To communicate with a particular device,
such as the one shown in Figure 4 with a
GPIB address of 13, the user double-clicks the
"GPIB1::13::INSTR" line. After doing so, the
screen as shown in Figure 5 appears.

The blue "Template" tab exposes all of
the attributes of the GPIB interface, only
one of which must be changed to deal with
Anritsu's few non-compliant instruments.The
viSetAttribute tab contains a pull-down list of
Attributes that can be selected, including the
VI_ATTR_TERMCHAR_EN attribute.This
attribute defaults to VI_FALSE and must be
changed to VI_TRUE.

Our non-compliant instruments are equally
non-compliant whether transmitting or receiving
on the GPIB bus.The instruments ignore the
EOI when receiving a command, thus the
computer must append the line feed termination
character.Similarly, when the instrument is
responding to a query, the instrument does not
assert EOI on the last character and instead
appends a new line character (sometimes a
carriage return character precedes the
terminating line feed). In its default mode,
VISAic presumes the instrument will be
ending its transmissions with EOI.

By setting the VI_ATTR_TERMCHAR_EN
attribute to true, you are enabling the
termination of character reception when a
particular termination character is received.

One other attribute, VI_ATTR_TERMCHAR,
defines the EOS character, but since it defaults
to the desired line feed (0x0A), it need not
be changed.

After the attributes are properly set, click on the
blue "Basic I/O" tab, and then select the viWrite
tab.The VISAic screen will appear as Figure 6.

In the input text field named Buffer, type in
the command you wish to transmit to the
instrument. A very common command to use
is "*IDN?" which is defined in the IEEE-488.2
standard as an identification request.When the
instrument receives this command, it should
respond with a character string identifying the
instrument's manufacturer and model number.
The screenshot above shows the crucial
command termination character.

Following the "*IDN?" characters are the two
characters "\n".Veteran C++ programmers will
recognize these two characters, backslash
followed by lower case "n", as meaning new line.

(Figure 5)

(Figure 6)

When VISAic transmits the characters entered
into the Buffer field, it interprets the two "\n"
characters as a single new line (0x0A)
character.

(C++ and VISAic also interpret "\t"
as a tab character and "\r" as a carriage return
character.)

After entering the desired command
and new line into Buffer, the str ing is transmitted
to the instrument by clicking on the Execute
button.The screenshot shown previously was
saved immediately after clicking on Execute
and the Return Count field shows the number
of characters transmitted. Note that 6, not 7,
characters were transmitted, indicating that the
"\n" pair was indeed transmitted as a single new
line character.

To read the response from the instrument,
click on the viRead tab and then click the
Execute button.The screen will appear as
Figure 7 with the identification string from
the connected instrument.

Note that the string received from the instrument
is shown to have a "\n" (new line) as the
final character.

LabVIEW

When sending commands to a non-compliant
instrument using the LabVIEW language,
the line feed character is appended to the
command string as "\n" as shown in the Figure
8 graphic.

LabVIEW string constants default to "Normal
Display" mode in which the "\n" pair would be
transmitted as two characters.

If you right-click on the string constant,
as shown in Figure 9, a pop-up menu will
allow you to change the string from "Normal
Display" to "\' Codes Display" in which the
"\n" pair is interpreted as the single desired
new line character.

Summary

The difficulties communicating with a few
non-compliant Anritsu instruments can very
easily be resolved by simply appending a new
line character to every command and by
allowing responses from the instrument to be
terminated by a new line. Once these two rules
are understood and followed, the non-compliant
instruments are just as reliable and
communicative as our compliant instruments.

(Figure 7)

(Figure 9)

(Figure 8)

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Copyright © 2001 Anritsu Company
Specifications subject to change without notice.
Other brand and product names may be trademarks
or registered trademarks of their respective owners.

June 2001
P/N : 80202-00157
Printed in USA