From: jsm2r@poe.acc.Virginia.EDU (John S. Mitchell)
Subject: Supra v.32bis Disappointment (THE AUTHOR SPEAKS)
Organization: University of Virginia
Date: Fri, 10 Apr 1992 01:20:08 GMT


First of all, I want to thank sincerely everybody who responded 
with e-mail and netnews postings. Second, I can say that I did 
solve my problem with the Supra FAX Modem v.32bis connected to 
my Macintosh IIsi. This posting summarizes the solution that 
worked and other comments on the subject.

My problem, as previously posted: 

I connected my v.32bis modem to my IIsi's serial port using a 
Macintosh HARDWARE HANDSHAKING CABLE. 

	[Note: On the Macintosh side this cable has a 8-pin 
	minicircular connector, and on the modem side a 25-pin DB-25 
	connector. Pins 4 and 8 are jumped together on the Mac side (GND 
	and RXD+), while on the modem side pins 4 and 20 are jumped 
	together (RTS and DTR) and pins 1, 6, 9-19, 21-25 are not used 
	(the modem doesn't even use all of these). Basically the Mac 
	makes a compromise by jumping or throwing away certain signals. 
	Not perfect, but you do get hardware handshaking capability out 
	of it.]

I enabled hardware handshaking in my Mac communication application,
then I set my comm software to 57.6 kb/s for computer-to-modem 
(DTE) transfer speed, the theoretical maximum for v.32bis (14.4 
kb/s) with v.42bis compression (4:1).
	
	[Note: I realize this theoretical speed is basically 
	never achieved in real life. That doesn't make it wrong as a DTE 
	speed, since it will allow the modem to get whatever top speed 
	it can from its modem-to-modem (DCE) connection.] 

The resulting dial-up connections were marred by what I called 
"drop-outs." (Characters missing from words and lines of text.) 
Importantly, this occurred even if I turned off hardware 
handshaking and used software (XON/XOFF) flow control with the 
appropriate modem settings. Hence the cable could not have had 
anything to do with the problem.

At Supra Tech Support, I was told that the Macintosh serial port 
has a maximum transfer rate of 19.2 kb/s, and that my DTE speed 
cannot be set higher without the buffer overrunning, resulting 
in drop-outs. Supra said this had to do with the Mac's lack of a 
UART chip. True 57.6 kb/s transfers could only be accomplished 
on the Mac with a third-party add-in card, such as one model 
that adds multiple serial ports (supposedly with UARTs) to the Mac.

SUPRA WAS WRONG.

As good as their modem is, their tech-ies' knowledge of the Mac 
sucks. I heard from lots of people with various Mac models (Plus 
to IIci) who regularly use a DTE speed of 38.4 or 57.6 kb/s with 
no problem. In fact, the serial port's rated maximum is 230.4 
kb/s (or 0.920 Mb/s if externally clocked), at which LocalTalk 
networking runs. It is true that the Mac does not use UARTs, but 
the Serial Communications Controller (SCC) is quite capable of  
handling 57.6 kb/s.

	[Note: As some have suggested, very early Macs (pre-Plus) 
	may not be able to handle those speeds. (Missing the SCC, 
	perhaps?) Others mentioned that the when you use both serial 
	ports simultaneously on a Mac, and one is connected to a 
	LocalTalk network, then it is possible to overload the SCC 
	buffer. This seems pretty rare, depending on your network 
	traffic and whether the modem is connected to the "modem" serial 
	port or the "printer" serial port (because, it seems, the 
	printer port is "de-optimized" to avoid receiving data too 
	rapidly for the computer). I won't go any further into a 
	tangential subject I know very little about.]

Rather than the UART/SCC distinction, there is another Mac 
"feature" that was indeed the ACTUAL CAUSE of my drop-out 
problem. (Credits and kudos to Jack Kobzeff, jack@jato.jpl.nasa.gov,
for rescuing me from my plight.)

The problem was the Macintosh operating system (I use System 
7.0, but I would think earlier versions have the same problem). 
In short, it is too forgiving of background programs that want 
to take up processor time. (The Mac doesn't do true 
multitasking). Especially guilty are INITs and extensions, those 
pesky little programs that power users like to collect. They eat 
up so much processing time that high-speed data transfers can be 
affected. When data is coming in from the modem at 57.6 kb/s, 
the result was my now infamous drop-outs. 

Turning off the INITs and extensions (in System 7.0 by holding 
down <shift> on startup or restart) completely solved the drop-
out problem. In fact, I discovered that even loading a single 
one of my extensions resulted in drop-outs. Without extensions, 
they were gone. I did discover that with the suite of extensions 
I generally use, I could use 38.4 kb/s DTE connection speed 
without incident, better than the 19.2 kb/s Supra had said I 
must use. 

	[Note: This still seems kinda fishy, since some of those 
	who wrote me claiming "no problem" at 57.6kb/s MUST be using at 
	least some INITs or extensions. A possible reason is that their 
	hardware is somehow different (processor, clock speed, etc.). 
	Alternatively, I guess software could be written in a way not to 
	let background programs eat up as much processor time. (One 
	person wrote about a program assuming control of the whole 
	computer and effectively scheduling CPU time to the serial 
	ports.) I use the shareware Z-Term 0.9, an absolutely 
	outstanding program that still is not completely bug free.]

That's the long version of the whole sordid affair. Thanks again 
to everybody who offered their knowledge and assistance. (It's 
been fun being the instigator of such a significant netnews 
thread.) I encourage additional commentary by e-mail or news 
posting.

John Mitchell (jsm2r@Virginia.EDU)


P.S. I really do like the Supra v.32bis FAX Modem. It not only 
works well, it is a compact, smart looking machine with smart 
features. Even if a little more expensive than other v.32bis 
modems, the Supra's extras are IMHO worth it.