Stanford Research Systems SR844, User Manual

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4-28 Data Transfer
SR844 RF Lock-In Amplifier
Important!
•
If data storage is set to Loop mode, be sure to pause data storage before
reading any data. This is because the points are indexed relative to the oldest
point, which is continually changing.
•
Do not query the Interface Ready status bit after sending the
TRCL?
command. This bit will not be set until the transfer is complete.
When using the RS232 interface the points must be read as binary bytes (no
checking for linefeeds, carriage returns or other control characters). Most serial
interface drivers are designed for ASCII text only and will not work here. In
addition, the data transfer does not pause between bytes. The receiving interface
must always be ready to receive the next byte. In general, using binary transfers on
the RS232 interface
is not recommended.
FAST(?){i}
The
FAST
command sets the Fast Data Transfer mode on and off. The parameter
i
selects On (
1
) or Off (
0
). In the Fast Data Transfer mode, the values of X and
Y are automatically transmitted over the GPIB interface whenever data is sampled
and stored. The fast transfer mode is
not available over RS232 . The sample rate
sets the frequency of data transfers (512 Hz maximum). It is important that the
receiving interface be able to keep up with the transfers.
Offsets, Ratios and Expands are included in the values of X and Y. The values of
X and Y are transferred as signed integers, 2 bytes long (16 bits). X is sent first,
followed by Y, for a total of four bytes per sample. The values range from –32768
to +32767. The value of
±
29788 represents
±
full scale (i.e. sensitivity/expand).
Important!
•
At fast sample rates, it is important that the receiving interface be able to keep
up. If the SR844 finds that the GPIB interface is not ready to receive a point,
then the fast transfer mode is
turned off and GPB bit in the Error Status
Register is set.
•
The transfer mode should be turned on (using
FAST1
) before storage is
started. Then use the
STRD
command (see below) to start data storage. After
sending the
STRD
command, immediately make the SR844 a talker and the
controlling interface a listener. Remember, the first transfer will occur with the
very first point.
STRD
After using
FAST1
to turn on Fast Data Transfer, use the
STRD
command to start
the data storage.
STRD
starts data storage after a delay of 0.5 s. This delay allows
the controlling interface to place itself in the read mode before the first data points
are transmitted.
Do not use the
STRT
command to start the scan. See the programming examples at
the end of this section.