26 Feb
2010
26 Feb
'10
11:08 a.m.
The Model 4 was very similar to the model 3, but had a
CTRL key on the
For suitable values of 'similar' :-). The case was much the same (apart
from the colour), the floppy drive controller, RS232
interface, power
supplies, and monitor PCB were the saem boards. So were the drives
(officially 35 cylinder, single sided, but most would do 40 cylinders).
The keyboard, as you say, was similar, but it had a few more keys (CTRL,
CAPS, and I think f1-f3) put in 'spare' positions in the keyboard matrix.
The processor board was totally different. The major changes were the
video system (which was hard-wired TTL on the M1 and M3, and used a 6845
on the M4 together with twice as much video RAM, thus allowing 80*24 text
raher than the 64*16 of the earlier machines) and the memory addressign
hardware (the M4 supported up to 128K of bank-switched RAM, the BASIC
ROMs could be mapped out as you said, thus allowing a pure RAM memory MAP
for CP/M, etc).. A minor change was the signle-bit sound output on the
M4. This was mildly crazy, actually, The sound connector (a 4 pin header)
carries +5V, ground, a decoded output strobe and one of the data lines.
The sound board cotnaisn a 74LS74 dual Dtype (only one half used as a
single-bit output port) and the speakekr driver. The crazy thing is that
right next ot that header on the processor board is a 74LS74 with only
one half used.. Why the other half wasn't used for the sound output is
beyond me.
Eventually RS bought rights to LDOS, and released it
as "TRSDOS 6.0" for
Model 3 and 4, and Randy Cook finally started getting royalties!
I thought TRS-DOS 6.0 was for the M4 only, and was essentially LDOS from
the very beginning. You can run M3 OSes on the M4, but you con't get
access to the extra feauters of that machine.
-tony