Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!rice!sun-spots-request
From: ultra!beau@ames.arc.nasa.gov (Beau James)
Newsgroups: comp.sys.sun
Subject: Re: sun3/60 with apple laserwriter IINT
Keywords: Software
Message-ID: <205@brazos.Rice.edu>
Date: 18 Jul 89 20:38:15 GMT
Sender: root@rice.edu
Organization: Sun-Spots
Lines: 38
Approved: Sun-Spots@rice.edu
X-Sun-Spots-Digest: Volume 8, Issue 75, message 8 of 15

-- In SunSpots v8n69, kolb@nisc.nyser.net (Christopher Kolb) writes:

-- of the suggestions i received, some only might have helped ...
-- while others were flat wrong ... 
-- 
-- others however were on target.  the handshaking should be set to XON/XOFF,
-- *not* DTR.  this is because the sun is not fast enough to handle the
-- interrupts generated by hardware handshaking.  

This particular comment is flat wrong.  The speed of the Sun is not an
issue.  Lack of software support in the driver (and bugs) are the problem.
"Hardware handshaking", usually using the CTS signal line, need not
generate any interrupts at all.  

There are two directions of flow control: the external device asking the
Sun to stop sending, and the Sun asking the device to stop.

CTS flow control for the Sun-to-device direction is implemented by the
serial I/O chip hardware, at least for the chips used on the Sun CPU board
(Zilog 8530 SCCs).  When the device driver initializes the chip properly,
then CTS going away (going high) automatically stops the transmitter on
the chip.  (The driver can also ask the chip to generate an interrupt on
CTS transition, if the driver really wants to know about it.)

For the device-to-Sun direction, the Sun flow control software has to know
that it should raise the RTS hardware line rather than send an in-band
"stop" character (^S), and later drop RTS rather than send a "start" (^Q)
character.  The driver did not offer that capability in release 3.x, and
it didn't work in 4.0, but it is fixed by YAPT5.5c and in 4.0.3 (or so
we've been told by Sun).

The USARTs and UARTs used on other boards, e.g. ALM-1s and ALM-2s, have
similar capabilities.  Of course, the CTS & RTS signals have to be
properly cabled from the chip to the device, which is not always the case
(as with the "default" cable wiring configuration of the ALM-1).

Beau James				beau@Ultra.COM
Ultra Network Technologies		{sun,ames}!ultra.com!beau