slow serial I/O speed with USB modem

[Mietek]

We having a problem with Zoom 2985C USB modem (allocated by Win2000 as an additional COM port)when using our LabView application with VISA functions: responses are received only in shorts bursts of 12 -20 characters at the time.
The same modem responds very quickly with large volume of data (e.g. to AT&V1 command)when used with Hyperterminal in the same system (same 115200 Bd setting).

On the other hand some standard serial port modems operate well with our application, either connected directly to COM1 or when connected via an USB-to-RS232 adapter - and so using allocated additional virtual COM port (not the COM1 or COM2).
Any suggestions?

[Tom.]

Hello,

You may want to look at these KnowledgeBase articles:

Using Virtual COM Ports or Third-party USB-to-Serial Adapters with VISA in MAX

Commands Needed to Communicate with a Modem

This may give you an idea on why certain modems work and some do not with different types of serial communication. Do other modems work correctly with the same code?

Tom | NIUK

[Mietek]

Thanks Tom, I will have a look.

And - yes, several 'true' serial modems work well with exactly the same LabView application, just connected to COM1 or COM2 ports.
We have also tested one serial modem attached via popular USB-RS232 adapter and it worked well using this virtual COM port.

[Mietek]

Tom,

I did look at this KnowledgeBase article:
Using Virtual COM Ports or Third-party USB-to-Serial

But is so laconic that does not really help.

And note that the same modem responds very quickly with large volume of data (e.g. to AT&V1 command - over 600 characters)when used with Hyperterminal in the same system (same 115200 Bd setting).
So there must be something specific in VISA functions that makes it 'jerky'

Mietek