DISPlay:DIMMer?
wcl,
As stated, you cannot expect to send a random string to the device and receive it back, unless the operating instructions (contained in the device manual) explicitly state this possibility.
GPIB devices in general will respond to the commands defined in the IEEE 488.2 standard, as well as to the specific commands supported, each kind of device having its own set; once a command is sent, you may receive a response, depending on which command you sent and with what parameters. That said, you can find the definitions of supported commands and their response formats in the product manual. The API is just that set of supported commands, in case you might be wondering.
In the manual you found, chapter six contains all the necessary information for you to implement the command calls appropriately in your program. Things like the list of commands, as well as the format your device expects to receive these commands in, and the format of the response returned (if any) are all listed in enough detail in chapter 6.
For example, one of the listed commands specific to that device (the LTS-3900) is "DISP:DIMM", which as the lettering implies (DISPlay:DIMMer), allows you to control the device's display intensity, and even turn it off completely. It accepts one of three parameters ("HI", "LO", or "OFF") and expects the following syntax: DISP:DIMM<space><data>, where <space> is a white space, and <data> is one of the above parameters, verbatim.
The manual contains way more information than whatever you could get in these forums for the same amount of time. You should read it thoroughly.
