Stanford Research Systems SR844, User Manual

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4-8 Remote Operation
SR844 RF Lock-In Amplifier
Command Format
Communication with the SR844 uses ASCII characters. Commands may be in either
UPPER or lower case and may contain any number of embedded SPACE characters. A
command to the SR844 consists of a four character command mnemonic with optional ?,
arguments if necessary, and a command terminator. The command, arguments and
terminator may be separated by spaces. The terminator must be a linefeed <
LF
> or
carriage return <
CR
> on RS232, or a linefeed <
LF
> or EOI on GPIB. No command
processing occurs until a terminator is received. Commands function identically on GPIB
and RS232 whenever possible. Command mnemonics beginning with an asterisk ‘*’ are
IEEE-488.2 (1987) defined common commands. These commands also function
identically on RS232. Commands may require one or more parameters. Multiple
parameters are separated by commas (,).
Multiple commands may be sent on one command line by separating them with
semicolons (;).
There is no need to wait between commands. The SR844 has a 256 character input buffer
and processes commands in the order received. If the buffer fills up, the SR844 will hold
off handshaking on the GPIB and attempt to hold off handshaking on RS232. Similarly, the
SR844 has a 256 character output buffer to store output until the host computer is ready
to receive it. If either buffer overflows, both buffers are cleared and an error reported.
The present value of a particular parameter may be determined by querying the SR844 for
its value. A query is formed by appending a question mark ‘?’ to the command mnemonic
and omitting the desired parameter from the command. Values returned by the SR844 are
sent as a string of ASCII characters terminated by a carriage return <
CR
> on RS232 and
by a line-feed <
LF
> on GPIB. If multiple query commands are sent on one command line
(separated by semicolons, of course), the answers will be returned individually, each with
a terminator.
Examples of Commands
FMOD 1 <LF>
Set Reference Mode to Internal
FREQ 27E5 <LF>
Set the internal reference frequency to 27E5 Hz (2.7 MHz)
*IDN? <LF>
Query the device identification
STRT <LF>
Start internal data storage
OUTP? 1 <LF>
Query the value of X
Command Synchronization
IFC (Interface Ready, bit 1) in the Serial Poll status signals that the SR844 is ready to
receive and execute a command. When a command is received, this bit is cleared,
indicating that command execution is in progress. No other commands will be processed
until this command is completed. Commands received during this time are stored in the
buffer to be processed later. Only GPIB serial polling will generate a response while a
command is in progress. When all pending commands have executed, the IFC bit is set
again. By checking IFC with serial polls, a host computer can ensure that all previously
sent commands have finished before sending a new command.