Why does RapidPort fail where DigiRAS does not?

We produce time & attendance software. My area of concern is using modems to communicate with embedded-system time clocks. We have been communicating with 2400 & 19200 bps time clocks for years with our software using multiport DigiRAS products. When we switched to a USB Digi Rapidport, communications to the 19200 bps devices are working fine, but when dialing out to 2400 bps device, it comes back with “NO CARRIER” after about 4 seconds. Both are using the same phone lines (manually switched between DigiRAS & Rapidport as needed). The initialization string in use is “AT&F0E0W2” on RapidPort. The result code using “AT#UD” is 50, “AnyKeyAbort”. This is a software application, so there is no character output after the dial command for at least 60 seconds. I think our software can be ruled out, as the same software works fine with DigiRAS, with a different initialization string, of course.
What can I try? This error makes no sense.

Rapid port and DigiRAS are very different products under the covers. I’m guessing it might be with the way they handle the serial ports.

You could try a port monitor trace between them and find the differances: