Newsgroups: comp.dcom.modems
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!magnus.acs.ohio-state.edu!usenet.ins.cwru.edu!neoucom.edu!wtm
From: wtm@uhura.neoucom.EDU (Bill Mayhew)
Subject: Re: New v.42 Modem for $300
Message-ID: <1991Jun22.155311.8203@uhura.neoucom.EDU>
Keywords: v.42 Cheap
Organization: Northeastern Ohio Universities College of Medicine
References: <629@morpho.UUCP>
Distribution: comp
Date: Sat, 22 Jun 91 15:53:11 GMT

Something to consider is the UART chip.  I reviewed GVC v.32 / v.42
v.42bis modems recently.  The internal GVC modem uses an 80C450
UART chip which is a 40 pin surface mount device.  Given the
interrupt structure of the IBM-derived ISA bus, this does not bode
well for good performance at high speeds because an interrupt
signal will be generated for each incoming character.

In the MS-DOS world, you can hand code an assembly level driver to
mitigate the effect of receiving a large number of interrupts.
With a Unix system, the kernel will be forced to do a context
switch to process every interrupt.  Even on a 33 MHz '486 system
this may require many microseconds and imposes a significant
performance penalty.

The best solution is to have an external modem, buy a cheap comm
board and replace the stock xx450 UART with a National
Semiconductor 16550 UART.  The 16550 is compatible with xx450, but
may be switched to FIFO mode cutting interrput burden by as much as
94%.  The Western Digital version of the 16550 has bugs; do not use
it.  Of course, you have to supply the software driver for your
host.  Newer comm programs such as Procomm have integral support
for the 16550 UART.

The other solution is to find out if the xx450 chip is socketed on
the internal modem and replace it.  The only internal modem I am
certain of having a socketed 16450 is the Multitech v.32 modem; it
is much more expensive.

The GVC modem is available in external form for about $405 in
single unit quantities.

Bill


-- 
Bill Mayhew      NEOUCOM Computer Services Department
Rootstown, OH  44272-9995  USA    phone: 216-325-2511
wtm@uhura.neoucom.edu   ....!uunet!aablue!neoucom!wtm
via internet: (140.220.001.001)