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Chapter 6, COMPUTER OPERATION
6.4.02 General Features
6.4.03 Terminator, Status Byte, and Overload Byte
6.4.04 Delimiters
6-6
Readers should refer to the document "USB Drivers for the Model 7124 and 7270
Lock-in Amplifiers", available from the www.signalrecovery.com website, for
further information about installing these drivers.
Unlike the RS232 interface, there is no difficulty ensuring that the correct cable type
is used for USB operation. All that is required is a standard USB-A to USB-B cable,
which if the driver is correctly installed on the computer will then allow operation
without further adjustment.
Commands sent to the instrument over the USB interface should be terminated with
a null character (ASCII 0).
Responses from the instrument consist of a string that is terminated with one or three
bytes depending on the setting of the Output Terminator control on the USB Status
Menu – see section 5.3.19 – as follows:
Output Terminator = NULL
1) A null character (ASCII 0)
Output Terminator = STATUS
1) A null character (ASCII 0)
2) A byte representing the value of the Status Byte (table 6-1)
3) A byte representing the value of the Overload Byte (table 6-1)
In the case of commands that generate a single response, the controlling program
should send the null-terminated command string to the instrument and then read the
response bytes sent back from it until the null is detected. If the Output Terminator =
STATUS then two further bytes (the status and overload bytes) should then be read to
complete the transfer.
Commands that do not return data still always return the one or three terminator
bytes, allowing the controlling program to know that the command has been
implemented.
Data stored in the instrument's curve buffer can be transferred in two ways. In the
ASCII mode, each value is terminated in a null character until the last value, which is
then terminated as above, whereas in the binary dump mode each data point occupies
two bytes, with the complete dump being terminated as above.
In either case it is necessary to know how much data will be transferred, by sending
the M (monitor) command and reading the response, before initiating the transfer.
This is because in the ASCII dump mode the controlling program needs to "count"
each instance of a null character until all available points have been read, while in
the binary dump mode it needs to know exactly how many bytes to read, since the
data itself can contain null characters. Only by doing this can the program be
prevented from requesting more data from the USB endpoint than the lock-in has
actually sent.
Most response transmissions consist of one or two numbers (expressed as ASCII
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