From: geoff@zswamp.UUCP (Geoffrey Welsh)
Newsgroups: comp.dcom.modems
Subject: Re: Internal vs External modem peerformance
Date: Sat, 25 Jul 92 00:18:04 EDT
Organization: Izot's Swamp

harlan@netcom.com (Harlan Lau) writes:

> Is there any performance advantage for internal modems over
> external versions of the same modem?

   There's nothing inherently superior to either design; any differences are 
due to implementation, not architecture.

> Aren't external modems in some cases limited by the speed/type
> of the  uart on the serial line, which might not allow them utilize
> their full potential.

   Yes... and so are internal modems.  In fact, internal modems have the UART 
built right into them, so you're stuck with whatever comes on the modem.  Even 
if you buy a modem with a 16550 installed (or, better yet, some kind of 8250 
emulation but not a real 8250-type chip), you're stuck forever servicing an 
interrupt per byte (or per few bytes) from something that looks to your driver 
like an 8250 and therefore might as well be one from an interrupt overhead 
point of view.

   If you elect to install an external modem, your choices are many:

 - simple 8250 or 16450
 - 16550
 - Hayes ESP port
 - intelligent serial card, with drivers supporting efficient block transfer
 - Ethernet port connecting to a terminal server
 
 ... and probably many more that I haven't thought of.

> I used Microsoft`s MSD utility to establish that my Gateway PC has
> 8250 uart's on it's serial ports. Rather than upgrading to the 16550
> uart's it seems easier to get the internal version of the modem.

 ... with 8250 or 16450s on it, most likely.  Net gain, performancewise: 
zilch!

   Did you think that internal modems had some magical interface to the PC's 
I/O bus?  If so, how did you think they retained compatibility with 'standard' 
serial ports?

> Is there any reason to get an external modem other than being able
> to see the lights and being able to share the modem with other
> PCs?

 - Being able to use more efficient serial ports.
 - Being able to reset the modem without resetting your PC.
 - Being able to connect the modem to computers other than ISA PCs (including 
MicroChannel machines).
 - Not needing to take your computer apart to move your modem.
 - Saving a slot in your PC (by the time you put in your basic I/O cards, 
drive controller card, video card, mouse card because you've got modems on 
your serial ports, a SoundBlaster, a JT-FAX card, and another doohickey or 
two... you're really eager to keep away from devices which take up a whole 
slot and give you one modem, rather than eight or 16 serial ports).  
 - Hopefully, keeping phone line lightning strikes isolated from your PC (but 
don't count on it with cheap modems, which will happily spike the RS-232 lines 
rather than self-destruct dealing with the surge).
 - Or, how about the ability to free up a COM port for those once-in-a-blue 
moon LapLink connections without having to determine for the umpteenth time 
which jumper(s) on your I/O card re-enable COM2 and its interrupt request 
line?

Geoffrey Welsh, 7 Strath Humber Court, Islington, Ontario, M9A 4C8 Canada
geoff@zswamp.uucp, [xenitec.on.ca|m2xenix.psg.com]!zswamp!geoff (416)258-8467
Newsgroups: comp.dcom.modems