26 Apr
2007
26 Apr
'07
2:12 a.m.
Subject: Re: Hand-rolling a CP/M machine
From: "Chuck Guzis" <cclist at sydex.com>
Date: Wed, 25 Apr 2007 11:49:49 -0700
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at
classiccmp.org>
On 25 Apr 2007 at 13:24, woodelf wrote:
Perfectly reasonable, though at first it was a simple system.
The Z80 and 8257 (or 8237) coexist fairly well. There are some signal
differences for the glue TTL but it works. Back in the day it was also
popular to do that becase 8257 was cheaper and faster then Z80 DMA and
didn't have to be clocked synchronous like the Z80 DMA.
Allison
Well if anybody is doing VDM-1's the big
problem is that you can't find
the Character ROM any more. I like the idea of TTL display like VDM-1
but since this is not the late 70's a 80x24 screen is reasonable
and the data out is a simple VGA format, since the monitors are easy to
come by.
At the expense of getting booed off the topic, if you need simple
80x24 and are using something like an 8085, the 8275/8276 CRT
controller with an 8257 DMA controller (in auto-initialize mode)
doesn't take a lot of extra glue. The gotcha is that "special
effects" like bolding and underlining take up space on the display
(Doesn't the Wyse 50 do that?). There is a "transparent" mode, but
management of the screen buffer becomes a real headache. I've never
tried to interface an 8257 to a Z80, so I don't know if the timing
works there.