Re: RS232 or USB to RS485

From: Hans-Bernhard Broeker (broeker_at_physik.rwth-aachen.de)
Date: 02/13/04 

Date: 13 Feb 2004 09:35:43 GMT

Paul Keinanen <keinanen@sci.fi> wrote:
> On 12 Feb 2004 15:35:24 GMT, Grant Edwards <grante@visi.com> wrote:

> >That's what I've been told the Windows serial driver does: It
> >checks the shfit-register empty status bit every millisecond
> >until true, then drops RTS.

> If that is really true, then this is a nice feature if you are
> operating at 1200 bit/s or below, in which case you are probing every
> bit, but at 115k2, one milliseconds is more than 11 character times
> and in the worst case the master (PC) transmitter is on, while the
> slave is already sending the 11th byte of the response :-).

Well, that's what you get for trying to force a piece of hardware
designed for one line discipline (like the 16550 is for RS232), driven
by a system that isn't anywhere near usable as an RTOS, to talk an
inherently incompatible language at its full native speed. That's a
bit like handing a blacksmith a sledgehammer and expecting him to make
you a wedding ring.

If you're building an embedded system you can patch up the defects
with some additional circuitry and sufficiently responsive pieces of
software --- but that would invalidate the whole reason for putting a
16550 instead of a dumber kind of UART on the board: the luxury *not*
to have to do all the nitty-gritty bit-fiddling of control lines in
software, and *not* to have to reliably handle any interrupt request
within on byte-time of the serial link.

So, in a nutshell: yes, some UARTs can drive both RS232 and RS485,
either by design or with a bit of glue logic in hardware and software.
But no, your typical 16550 (or work-alike core) found in a PC running
Windows is *not* in that category. 

-- 
Hans-Bernhard Broeker (broeker@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

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