I got this from Xircom Technical Support regarding my PC Card modem..

> Xircom emulates an 8250 UART interface on the CEM28/CEM2 product
> lines. Because this is only an emulation, it means the serial
> bottlenecks inherent in UART chips do not apply.  The 8-bit data path
> is connected directly to the PC Card modem and does not go through
> serialization, but instead uses a parallel method of transferring the
> data to the modem. The only similarity to an actual 8250 UART chip is
> the addressing scheme and bit definitions for things such as status
> and interrupt control. This allows the data path to the PC Card modem
> to operate very much like a LAN adapter running in I/O mode.

As I understand it, the 8250 raises the interrupt for every byte that it
receives whereas the 16550 with its 16 bytes buffer can be set up to
raise the interrupt 1/16 as frequent. (Aside: Is there a proper term for
this?) It seems the above scheme in the Xircom doesn't overcome the
"problem" of the CPU having to pay attention to the UART for every byte
that it receives; even so, I have not experienced any data overruns
under Win95. Does anyone know what trick is used to overcome this? (I
believe the Banksia (ala Microcom) PC Card modem uses a similar scheme).

Interestingly enough, there is an undocumented S-register (134) which
controls the UART FIFO Threshold. I wonder if this is the register that
can be set so that the proprietary buffering scheme can be set to raise
the interrupt as frequent as the 16550 rather than the 8250. (Xircom
warns me not to change anything but I have done so without any adverse
effect. They won't tell me how to use it :-(. I'll bug them a bit more.)

Anyone care to comment on the last bit about the PC Card modem running
like a LAN adaptor?

Regards.

mslim{at}ne.com.au

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