Path: utzoo!utgpu!watserv1!maytag!xenitec!zswamp!root
From: root@zswamp.fidonet.org (Geoffrey Welsh)
Newsgroups: comp.dcom.modems
Subject: UARTS for speed...
Message-ID: <5073.274CAD27@zswamp.fidonet.org>
Date: Thu, 22 Nov 90 18:27:48 EST
Organization: Izot's Swamp BBS - Kitchener, Ontario

Cliff Yamamoto (cyamamot@kilroy.jpl.nasa.gov ) wrote:

 >I've been running my serial line at 38.4Kbaud on my 386 clone for a few
 >weeks now without any problems (not running Windows or Deskview).  However,
 >I've heard that one can change the UART to offload some work on the CPU.

   It may look like I'm contradicting what I said before (specifically, that a 
16550 won't help anyone who's not already having problems with an 8250 or 
16450) but I'm not, really:

   The 16550 will let you trigger receive interrupts at various receive FIFO 
levels (up to 14 bytes) and write up to 16 bytes when the transmit FIFO is 
empty. So... *IF* the software is configured to do it, installing a 16550 
*can* reduce CPU overhead slightly.

   However, you won't feel the difference. (That's why I'm not really 
contradicting what I said before).

 >Can anyone tell me if the 8250, the 16450 and the 16550A are pin-for-pin
 >interchangable?  I'm not sure if I have the 8250 or the 16450, but I'd like
 >to eventually plug in a 16550A.  Also do all three share the same port and
 >programming characteristics?  Are 16550A readily available?

   All three are pin compatible. The 16550 powers up in 16450 mode, for all 
programming intents identical to to the 8250. To activate the FIFOs, you write 
to a read-only status register. I'd like to give full specs, but Matthew 
Desmond (matt@spinq.uucp for flaming purposes) has my National datasheets.

   Here's a quick DOS Turbo Pascal program I hacked together to enable the 
FIFOs on a 16550 and set the trigger level to 1 byte (which won't lower your 
overhead but it will make sure that you stop losing incoming bytes):

PROGRAM NS16550A;

VAR
   address: ARRAY[ 1 .. 4 ] OF word;
   portnum: integer;
   temp: string;

BEGIN
   WriteLn;
   WriteLn('NS16550A Buffer Control ((C)) 1989, 1990
            by Geoffrey Welsh, v901010');
   WriteLn;
   address[1] := $3fa;
   address[2] := $2fa;
   address[3] := $3ea;
   address[4] := $2ea;
   IF ParamCount = 0 THEN BEGIN
      WriteLn('Enables or disables FIFO buffering on a specific COM port');
      WriteLn;
      WriteLn('USAGE: NS16550A COM{1|2|3|4} {ON|OFF}')
      END { IF ParamCount }
   ELSE BEGIN
      temp := Copy( ParamStr(1), 4, 1 );
      portnum := Ord( temp[1] ) - 48;
      IF (portnum < 1) OR (portnum > 4) OR (Length( ParamStr(1) ) <> 4) THEN
         WriteLn('Improper port # given')
      ELSE BEGIN
         IF (ParamStr(2) = 'on') OR (ParamStr(2) = 'ON') THEN BEGIN
            WriteLn('Enabling buffer...');
            Port[ address[ portnum ] ] := 1;
            IF Port[ address[ portnum] ] < 64 THEN
               WriteLn('There doesn''t seem to be a 16550 at COM',
                        temp[1], '!')
            END
         ELSE IF (ParamStr(2) = 'off') OR (ParamStr(2) = 'OFF') THEN BEGIN
            WriteLn('Disabling buffer...');
            Port[ address[ portnum ] ] := 0
            END
         ELSE
            WriteLn('Improper on/off instruction given.')
         END { IF portnum ELSE }
      END { IF ParamCount ELSE }
END.

   If you're serious about lowering your overhead, you might want to invest in 
a smart serial card. Mind you, they're a lot more expensive that 16550s 
(hundreds of bucks vs. $25!)

 

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