Subject: Re: Serial port issues on Sun4/260
To: None <bwalker@musings.com, dgilbert@pci.on.ca>
From: Chris Torek <torek@bsdi.com>
List: port-sparc
Date: 01/16/1996 14:41:39
>The Zilog chips that are used in so many of Sun built-in serial
>ports are not capable of automatic flow control.

This is almost right (or slightly wrong) :-)  There is a bit in
write register 3 called `auto enable'.  From /sys/sparc/dev/zsreg.h:

/*
 * Bits in Write Register 3 (`Receive Parameters and Control').
 * Bits 7 and 6 are taken as a unit.  Note that the receive bits
 * per character ordering is insane.
 *
 * Here `hardware flow control' means CTS enables the transmitter
 * and DCD enables the receiver.  The latter is neither interesting
 * nor useful, and gets in our way, making it almost unusable.
 */

Thus, if you are sending from the zs chip to a modem, the modem
can drop CTS to get the zs chip to shut up for a while, but the
zs chip will never wiggle any signal to get the modem to shut up.
This means you must never overflow the zs chip's fifo.

Chris