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Path: utzoo!linus!philabs!cmcl2!acf4!tsc2597
From: tsc2597@acf4.UUCP (Sam Chin)
Newsgroups: net.micro.pc
Subject: Re: Data General One Serial Ports
Message-ID: <1050025@acf4.UUCP>
Date: Fri, 26-Apr-85 09:15:00 EDT
Article-I.D.: acf4.1050025
Posted: Fri Apr 26 09:15:00 1985
Date-Received: Sun, 28-Apr-85 05:04:56 EDT
References: <817@unmvax.UUCP>
Organization: New York University
Lines: 22

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I have used the DG One but know nothing about the internals. However, the
difference between the 8250 and the 8251 is that the 8251 gets its clock
from an external timer such as the 8253. The 8253 has normally 3 counters.
On the PC only one is available (the others are used for the speaker etc)
for a real time clock interrupt and is wired to the 8259 interrupt
controller which is then multiplexed into the 8088. Since the DG One is
fairly compatible with the PC, I would assume that the 8259 is at the same
port location and that it uses the same flag as on the PC. You can find out
what port your 8251 output is connected to by outputting a byte to each of
the 255 ports until you get something coming out (open your drive doors first
because you might mess with the disk i/o ports). Normally the status port is
the previous or next port address. To set the baud rate, you probably have
to figure out which counter the DG uses to put the divisor on the 8253. It
is possible that there are more than 1 8253's or a real time clock chip.
Hopefully the crystals are at the same speed. I have succesfully brought up
file transfer programs on antique Z-80 machines this way. Good Luck.

                                       Sam Chin
                                       allegra!cmcl2!acf4!tsc2597
                                       tsc2597.acf4@nyu