asterisk and other characters at the end of GBIB command string

[pauls]

Could somebody explain 2 strange observations communicating with an instrument (MKS 467 flow controller)via its GPIB interface:

1) sending command string e.g. "ON 1" as listed in the instruments manual (open valve 1) with CR at the end does not work; however, adding an asterisk at the end "ON 1*" works!

2) when reading data from instrument, e.g. "FL 1" (read channel 1 flow), the manufacturer's command above does not work. Nevertheless, "FL 1?*" works, but in a strange way: it reads flow, but when swithced to read another channel "FL 2?*", it continues to read the "old" channel 1 about 3-6 times, then finally it starts reading the new channel 2. How to make the switch immediate?

I know that majority
of these instruments were produced with serial interface, but I have one with IEEE one. Maybe this has something to do with what I observe? VISA commands have identical response as above. Any comments are greatly appreciated.

[DiegoF]

Hi,

IEEE 488.2 is backwards compatible with IEEE 488.1 in the electrical and the low level operations of the bus (such as handshake, data transfer, etc). The main addition of 488.2 to the standard is the status reporting, a high level message exchage state machine and some standard data commands.

If there are no GPIB errors, we can assume that the communication is working properly. I would suggest looking at the instrument's user manual or contacting the manufacturer for detailed information on what the instrument is expecting.

You can check the GPIB interface properties and verify the termination character settings. You can see there isn't too many settings that can affect the communication in such a way.

Hope this helps.

DiegoF.
National I
nstruments.