pete@octopus.UUCP (Pete Holzmann) (01/12/88)
BACKGROUND:
Recent discussion in comp.mail.uucp suggests that a reasonable number of
people find uucp gets into trouble when running faster than 4800 baud, due
to loss of incoming characters.
We fall into that category, and, as I'd like to be able to get the fullest
possible performance out of our Telebit modem, I've checked out the problem
pretty carefully. Now that I know we aren't alone, I'm wondering how
extensive these problems really are! But first, a little slightly technical
background info (if you don't care, skip forward a page)...
In order to avoid the problem, your system must be capable of:
- accepting 3 full packets of incoming uucp data at full speed
without any flow control needed (around 220 characters?)
or
- providing correct hardware handshake end-to-end
Systems that are capable of handling high speed input without hardware
handshaking generally have some sort of separate serial I/O processor that
can always keep its FIFO or silo clean (no big interrupt-disabled delays).
Correct end-to-end hardware handshaking is much more hardware dependant. It
usually comes down to a question of (1) how long interrupts can be disabled
on the processor that talks to the UART, and/or (2) which particular UART
chip is used.
Our system is a good example of a computer that is likely to get in trouble,
although it didn't seem so at first. While we have no separate I/O processor,
our UART chips are the 68681, which have a 4 byte input FIFO, with on-chip
hardware handshaking support. Unfortunately, the hardware handshaking is
slightly wedged: the line doesn't drop until the FIFO is *full*, with room
for only one more incoming character (in the shift register). Thus, unless
the sender can stop within one character time, you'll lose a character
whenever the FIFO fills. Connections 68681 <-> 68681 are fine. But (for
example), the popular Intel 8250 type UART is typically used such that
2 characters may be sent after the handshake line drops.
Thus, we have trouble receiving data from a Telebit modem, but not from
other computers that use the 68681. Telebit is considering (at a low
priority, 'cuz they haven't had other complaints) making 1-character-runout
another option. This would help anybody having this problem, at the expense
of somewhat slower throughput.
Soooo. A SURVEY. I'll tabulate any results and report to the net if there
is interest (please mail to {hpda,pyramid}!octopus!pete):
-------------------------- cut here ----------------------------------------
Fast UUCP connection trouble survey. Duplicate as you wish.
Your name =
A good uucp address for you, if any =
Host device =
(computer and/or I-O processor/UART)
Device that you connect to via UUCP =
No-trouble baud rate (maximum) =
Maximum baud rate available for this link =
(if any speed at all would work)
Comments (if any):
------------------------- cut here -----------------------------------------
Example (us):
Your name = Pete Holzmann
A good uucp address for you, if any = pyramid!octopus!pete
Host device = Stride 68010 box, UART = 68681
Device that you connect to via UUCP = Telebit modem
No-trouble baud rate (maximum) = 4800
Maximum baud rate available for this link = 19200
Device that you connect to via UUCP = PC/AT
No-trouble baud rate (maximum) = 9600
Maximum baud rate available for this link = 38400
Device that you connect to via UUCP = PC/AT (Procomm file transfers)
No-trouble baud rate (maximum) = 4800
Maximum baud rate available for this link = 19200
Comments (if any):
UUPC on a PC seems to do a little better. It must watch the PC's UART more
carefully.
Running file transfer protocols to upload data from a PC, using Procomm,
gets me in trouble too: Procomm does not support hardware flow control at
all! (And none of the common PC protocols like X/Y/Zmodem support character
based flow control either)
THE END. Hope this turns into something useful for all of us!
--
OOO __| ___ Peter Holzmann, Octopus Enterprises
OOOOOOO___/ _______ USPS: 19611 La Mar Court, Cupertino, CA 95014
OOOOO \___/ UUCP: {hpda,pyramid}!octopus!pete
___| \_____ Phone: 408/996-7746