Error!!! 2 instruments for 1 GPIB slot

[magou]

Hi,

I using Measurement & Automation Explorer Version 3.1.1.3004. I encountered some problem when connecting 2 instruments in one GPIB slot. Whenever i click on "scan on instruments", it seems like cannot detect 2 instruments at the same time. But when i connect only one instrument, it is able to detect that instrument all the time. 2 instruments use different addresses. there is only  1 GPIB slot.

Is there any settings i need to set in order to detect 2 instruments all the time with only one GPIB slot?

pls advice

thanks.

[Dennis_Knutson]

There is nothing that you should need to do to detect multiple instruments. When you have them both connected, does it always detect the same one? What are the makes and models of the instruments?

It's possible that a defective cable can cause this or one of the instruments is bad. Try a different set of cables first and if you have different GPIB instruments to swap, try that as well.

[magou]

Ok. when 2 instruments are connected. when i click the "scan for instruments", it also come out different types.

Sometimes it can detect only one and sometimes both.Sometimes worse, it can detect up to 5 instruments where i only got 2 instruments connected to it. I believed this is the unstability.

I had tried to connect my 2 instruments on another computer. The problem is the same. From there, i believed that there is nothing wrong with the GPIB card. Also nothing wrong with the cables as i can detect them whenever i connect only one instrument (always stable).

so you mean that i do not need to edit anything for my settings even though i had put mulitple instruments on one GPIB. But now, it seems like the instabilty occurs when i connect 2 instruments. one instrument no problem.

Instrument 0 - address 5

instrument 1 - address 17

[Dennis_Knutson]

No, there is nothing to set to make a GPIB controller detect multiple instruments. It is always supposed to be able to detect multiple instruments. What you can do is go into the controller properties and change the timing. Some instruments (and especially old ones), benefit from slowing down the bus timing. You haven't provided the make and models of the instruments like I asked so I have no idea if they are supposed to be compliant to 488.2.

Since you don't typically have to do a scan for instruments very often, this may be a situation that you will just have to live with. What will need to be fixed will be if you have problems communicating to each individual instrument when you write your program.

Also, there can be cable and instrument faults that will allow you to detect a single instrument but fail when multiple are connected.

[magou]

Sorry, i forget the models and makes of the instruments. One of them is a HP meter.

So what is the suitable range of timing to set? I will try again.

[Dennis_Knutson]

I don't have a controller here and don't remember the actual options. Right click on the device in MAX and select properties. One one adjusts bus timing (not the timeout setting). The default setting is for the minimum time. Change to the max time you can and see if that makes a difference.

One thing I haven'tt asked is the model of the GPIB controller. I've used old is, pci, and usb and I don't think it would make any difference but you never know.

[magou]

Mine is using  PCI. My current timing is 1sec. I had tried 10sec but still useless. Maybe i will try the to set to the max timing.

If still unable to solve the problem, do you think i should add another GPIB card into the computer. So that one instrument is connected to one GPIB card?

[Dennis_Knutson]

If you set a timing of 1sec or 10sec, you set the timeout for a read. There is a timing setting in the order of usec that you can experiment with.

The GPIB spec calls for up to 15 instruments on a bus at one time. I see no reason to use two different controllers unless you are unable to fix the problem. You have a bad controller, cable, or instrument. That is why I suggested swapping out the cables and instruments - in order to isolate the issue.

[Steven_T]

magou,

I agree with Dennis.  The problem is can be coming from either the cable or some old instruments.  The "scan for instruments" action will cycle through all GPIB addresses and address each one to listen.  If there is an instrument at one of these addresses, it automatically asserts its handshaking lines.  The controller will then try to communicate with the instrument by sending a fairly common command (*IDN?). 

Your instrument may be so slow that it shows up on multiple addresses (because it never un-asserts the hanshaking lines).  What version of the NI-488.2 driver are you using?  I noticed that the newer versions of the driver do not wait nearly as long for each address that it checks.

Thanks,
Steven T.

[magou]

hi all,

thanks for your advice. i tried to change computer and cables. still the same. so i believe it does not lies with the cables or computer. As for the verison of NI-488.2 driver, where can i check? I also believed that my instruments are old.

Is there any other ways to delay the timing? sorry i able to print screens the problem i encountered.