XT2640 Operating Manual
13 July 2016
Page 138 of 187
24.1.4.3.3
Register
and
Command
Differences
to
Other
Interfaces
Some registers and commands are different in the GPIB to those for the other interfaces, and some are only available from the GPIB
interface.
The LOCAL and LOCKOUT commands are silently ignored if received via the GPIB interface.
The ESR register has a different format than for the other interfaces, for the GPIB interface its contents are as defined by IEEE488.2.
The *ESE, *ESE?, *SRE, *SRE?, *TST? and *WAI commands (as defined by IEEE488.2) are available from the GPIB interface but are
unavailable for the other interfaces.
For all other interfaces if a query command is sent to the XT2640, and then another query command is sent to the XT2640 in a
different set of commands before the response of the prior query has been fully transmitted by the XT2640 then an error is raised
and the second query command is discarded. For the GPIB interface if this occurs then an error is raised and the response to the
second query command replaces that of the 1
st
query command. This is as required by IEEE488.2.
24.1.4.4
IEEE488.2
COMPATIBILITY
The XT2640 is largely compatible with mandatory portions of the IEEE488.2 standard, however there some deviations.
The XT2640 does not lockup the GPIB when requested to talk and there is no prior query command response to transmit,
as required by the IEEE488.2 standard; as described above the XT2640 responds with a blank response in this case.
The XT2640 does not use a whitespace character as a command or data separator as required by the IEEE488.2 standard;
in most cases whitespace characters are ignored by the XT2640.
The “pon” bit of the ESR register has an extended meaning in the XT2640 beyond that required by IEEE488.2.
24.2
COMMAND
SYNTAX
All commands to the XT2640 use the standard 7‐bit ASCII character set using 8‐bit encoding (the 8
th
bit is zero) independent of the
actual interface being used. A command is a stream of characters, the XT2640 storing received characters until a command
terminator character is received and only then is action taken on the commands. Further characters may be received while the
XT2640 is taking the actions needed for a preceding command but no action will be taken on them until all pending command
decode activity is completed.
Each command is a KEYWORD field defining the command possibly followed by further fields which refine the action of the
command. The available command keywords and the fields required for each are described in tables later in this section.
More than one command can be present in a single command set, in which case each command is separated from the previous by a
command separator character. If an error is found in any command within a command set then that command and any remaining
commands which follow it in the command set will not be actioned.
Since the interface is based on streaming ASCII characters the use of separator and terminator characters is required to ensure that
the extents of each field can be established. To improve the readability of commands you may also wish to employ whitespace
characters to spread apart fields.
24.2.1
SPECIAL
CHARACTERS
Certain ASCII characters serve a special purpose as described below.
24.2.1.1
COMMAND
TERMINATOR
CHARACTERS
The end of a command set is determined by the presence of a command terminator which may be the line‐feed, carriage return, form
feed or NULL (0 value) ASCII characters.
Everything between successive command terminators is a command set. A command set is limited to a maximum of 4095 characters
in total. There is no action taken or error generated if a command terminator is immediately followed by another command
terminator.
24.2.1.2
COMMAND
SEPARATOR
CHARACTER
If more than one command is in a command set then each successive command is separated from the previous by a command
separator which is the semi‐colon ASCII character (;).
Everything between successive command separators or command terminators is a command. There is no action taken or error
generated if a command separator is immediately followed by another command separator or a command terminator.
24.2.1.3
FIELD
SEPARATOR
CHARACTER
Most commands require command fields which refine the action of the command; each field is separated from the previous by a field
separator which is the comma ASCII character (,).
Everything between successive field separators, command separators or command terminators is a field.
24.2.1.4
SUB
‐
FIELD
SEPARATOR
CHARACTER
In some cases a single command field is made up of several sub‐fields; each sub‐field is separated from a previous sub‐field by a sub‐
field separator which is the colon ASCII character (:).
Everything between successive sub‐field separators, field separators, command separators or command terminators is a sub‐field.
24.2.1.5
WHITESPACE
CHARACTERS
Most fields and sub‐fields can have one or more whitespace characters at the beginning and/or end. The space, tab and underscore
ASCII characters are considered as whitespace characters.
