What com port (the I/O address) and IRQ did the port replicator get assigned?
Most DOS communication programs were limited to three or four standard ports. Usually, you could have any one of four different ports, called Com 1, Com 2, Com 3, and Com 4. The only difference between these was the I/O address used. The “standard” was that Com 1, 2, 3, and 4 used addresses x’03f8’, x’02f8’, x’03e8’, and x’02e8’ respectively.
In addition, each serial port needs a hardware interrupt. Again the standard, such as it was, let Com 1 and Com 3 both use IRQ 4, and Com 2 and Com 4 use IRQ 3. Notice that with this setup, you could not use Com 1 and 3 simultaneously, or Com 2 and Com 4.
Windows, on the other hand, can assign all kinds of addresses and IRQs to a serial port, and call that port Com 1, even though it doesn’t use the address and/or IRQ that DOS thinks of as Com 1.
So what may be happening is that windows has given your port replicator an address that your DOS program can find, but the IRQ is funny. Short transfers may work, or X-ON/X-OFF handshaking may work, but when the port tries to signal the processor via the IRQ, the DOS program doesn’t read it correctly and the connection fails.
Also, the device you added may not actually be a fully functional serial port. Windows may be emulating a serial port. So if you boot to DOS this emulation is not active, and the device no longer responds as a true hardware serial port does.
So, check the documentation that came with the serial port and see if it specifically says that it works with DOS, or if it has a DOS device driver that needs to be loaded when booting to DOS. And keep in mind that a DOS driver may use or support different configurations that the windows drivers.
If it is supposed to work with DOS, then go into the Device Manager in Windows and see what resources the device is configured to use. Then check your DOS communication program and make sure it is set to use the same address and IRQ as the device is using. If it is using an address or IRQ that your DOS comm program can’t be set for (like IRQ 11, for example), you’ll need to try to change the settings in the Device Manager to ones that your DOS program can use. If you can’t change them, you may be out of luck.
If the device needs a DOS device driver to load, make sure that gets loaded, and check the configuration it uses, and then make sure your DOS comm program is set for those settings.
If neither of those is true, you may not be able to use that device from DOS. In that case, you’d need to return it and get a different one that is “legacy” compatible.