Stanford Research Systems SR830, Manual

Page: 100 / 177

5-17
Remote Programming
fers on the RS232 interface is not recommended.
The parameter i selects the display buffer (i=1, 2) and is required. Points
are read from the buffer starting at bin j (j
≥
0). A total of k bins are read
(k
≥
1) for a total transfer of 4k bytes. To read a single point, set k=1. Both
j and k are required. If j+k exceeds the number of stored points (as
returned by the SPTS? query), then an error occurs. Remember, SPTS?
returns N where N is the total number of bins - the TRCB? command
numbers the bins from 0 (oldest) to N-1 (most recent). If data storage is
set to Loop mode, make sure that storage is paused before reading any
data. This is because the points are indexed relative to the most recent
point which is continually changing.
TRCL ? i, j, k The TRCL? command queries the points stored in the Channel i buffer.
The values are returned in a non-normalized floating point format (with
the units of the trace). There are 4 bytes per point. Multiple points are not
separated by any delimiter. The bytes CANNOT be read directly into a
floating point array.
Each point consists of four bytes. Byte 0 is the LSB and Byte 3 is the
MSB. The format is illustrated below.
The mantissa is a signed 16 bit integer (-32768 to 32767). The exponent
is a signed integer whose value ranges from 0 to 248 (thus byte 3 is
always zero). The value of a data point is simply,
value = m x 2 (exp-124)
where m is the mantissa and exp is the exponent.
The data within the SR830 is stored in this format. Data transfers using
this format are faster than IEEE floating point format. If data transfer
speed is important, the TRCL? command should be used.
Do not query the IFC (no command in progress) status bit after sending
the TRCL command. This bit will not be set until the transfer is complete.
When using the GPIB interface, EOI is sent with the final byte. The points
must be read using a binary transfer (see your GPIB interface card soft-
ware manual). Make sure that the software is configured to NOT termi-
nate reading upon receipt of a CR or LF.
When using the RS232 interface, the word length must be 8 bits. 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 trans-
exp mantissa
16 bits 16 bits
0
byte3 byte2 byte1 byte0