[Coco] 16550 wasRe: RS232 paks

[Gene Heskett]

On Thursday 05 March 2009, Bob Devries wrote:
>Of course, I have no idea exactly what the THROUGHPUT was in CPS with that
>setup.
>
When I was working on rzsz-3.36 last, the optimized version everybody should 
have by now, using hardware flow control, could transmit to an amiga at around 
740 cps with both sides set to 9600 baud.  In fairness to the flow control 
problems in the 6551, the amiga could of course keep up with the coco so the 
flow control never activated to trigger the bug under those conditions.  So of 
course that bug never stuck up its hand in my tests.  That would only show up 
when the target machine was slower than the coco.

Conversely, with the amiga sending to the coco, the flow controls were used to 
slow the amiga down, and again the average speed was in the 735 cps area.  And 
it was sustained over files of nominally a megabyte.  Only noticeable slowdown 
was those created by the no-halt controller if I was writing to a floppy, but 
usually I used my hard drive or the ramdisk I wrote since my coco3 has a 2 meg 
kit in it and I can instantly (a few hundred milliseconds) set it up at up to 
about 1.7 megs max but usually for 1.5 megs.

x and y modem, with their much simpler and faster error checking, can run lots 
faster than that.  The speed killer in rzsz is the crc checking, done on a per 
byte basis as the byte comes in or goes out.  Someday, someone should convert 
that to checking just one 256 byte buffer at once, it should at least triple 
rzsz's speed in cps.

Those speeds above were obtained with the crc table lookup method.  Using 
rzsz's original crc method slowed it by a bit more than half, to around 330 
cps both ways, so that table lookup was a huge improvement right there.  Now 
it needs to be converted to doing that loop once, over a full or partial 
buffer.  I believe that would get it nicely above 960 cps (9600 baud).

Anyway, those are the figures to shoot at, have fun, the code _is_ out there.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
<``Erik> 18,446,744,073,709,551,616 is a big number