USB-232 being unable to support more than 1 device at a time??!!?

[Nach]

Hi.

I am working on a project which requires the DAQ of several devices.  There are 2 serial devices, Pico PT104 and Druck DPI 740, that are connected to my PC via a NI USB-232 adapter.  When I first tested the devices (connecting only one at a time), both worked fine no matter which one of the 4 USB-232 ports I connected it to.  But when I connected both devices at the same time and run my DAQ program, I couldn't get any data out of PT104.  At first I thought the problem might come from the device itself.  But when I disconnect the other device, the problem went away and I was able to receive data as usual.

Can anyone tell me why I can't connect both devices to the USB-232 at the same time?

p.s. The communication with DPI 740 is done simply via a serial read/write.vi example of Labview while the communication with PT104 is done by the program which uses the library functions provided by the manufacturer.

[Dennis_Knutson]

Is there any chance that you could use the VISA functions for both instruments instead of calling an external program? Windows does not allow two different apps access to the same serial port and maybe the external app is checking availabilty of all ports and not just the connected one before it starts to run.

[Nach]

No, I can't use both device at the same time using VISA functions either.  (PT104 is always shown as 'busy' in MAX.)

I don't really understand why these 2 devices would be regarded as 1 port as they show up as separate COM ports in my system device manager (and also in MAX).

[Dennis_Knutson]

The busy in MAX is probably occuring because the third party software is still running. It has to be shut down completely.

I didn't say that the two devices would be regarded as one port. What I said is that the possiblilty exists that the third party software is checking the availability of all ports before it runs. Seeing that one com port is busy, it may just be waiting to proceed until it continues and connects to the specific port with the other instrument. Without having access to the third party software, it's just speculation but I definitely know that two spearate VISA session can run in parallel.

[Nach]

This is very funny.  But I got another single port usb adapter to connect the DPI 740.  Now that there's only PT 104 connected to the USB-232, the instruments can work at the same time!

I know this is not the best way to solve the problem because all serial devices should be able to connect to the USB-232.  But I still can't think of a way to work that out....

[Steven_T]

Nach,

At least you have a work-around for now.  You could use a program called port mon (port monitor) to monitor the activity on a serial port.  This will capture all accesses that interact with the ports.  If you specify port mon to monitor all ports, you can run your program to see if it tries to access ports other than the one connected to your instrument.  You will probably find that the program ties up several ports (very unfriendly behavior, as you have seen).

As long as a port is busy, other programs will not have access to it.  Let us know if this is not the problem.

I hope this helps,
Steven T.

[NANROC]

podrias decirme como conectaste el dpi740 al pc, lo he intentado pero no recibo datos, el puerto esta bien, pero al momento de correr el ejemplo read write serial de labview no recibo nada

agradecida de antemano por alguna respuesta