15 Mar
2007
15 Mar
'07
2:34 a.m.
One class of terminal that seems to have disappeared completely from
history is the viewdata terminal. 40x25 lines, colour and block
graphics and other effects implemented by control codes that took up
visible spaces..
Designed in the late '70s for display of the GPO (later British
Telecom) Prestel and other compatable dial-up services on residential
TV sets via integrated electronics (very few ever made due to cost),
it ended up as mostly dedicated terminals or set-top-boxs. The
service only really took off outside limited vertical markets in the
late '80s when "Micronet 800" started selling modems and (mostly quite
bad) terminal emulators for the various home micros. I used a few
dedicated terminals at the time, connecting either to the proper
service (run on primes I think) or our local editing system (a PDP 11
of some flavour.)
I've never seen an emulation that could cope with the rare case of
"remove the double-height -start-code" properly[1] apart from the one
I wrote, probably because nobody ever thought hard about the
implications of it..
I vote for the MAME-style hardware emulation approach!
Rob
[1] Because control codes took up display spaces, it was possible to
go back and overwrite them - this was often used for simple
animations, changing colours to hide or reveal things, etc, as at 1200
baud, it was quite slow enough to be visible.. Now.. double height
lines 'overflowed' onto the line below, hiding anything that was
printed there. when you remove the double-height start code, the
displayed double height characters are replaced by standard single
height characters, and the line below then appears. Now, if that line
had a start-double-height code on it, ignored up until now, then it
would suddenly appear and expand to cover line 3, hiding everything
that was there, etc etc... it was possible to swap an entire page of
text for another page of text, (all double height of course) by
overwriting a single control character at the top. Not bad when you
only had 1200 baud to send stuff. Unfortunately, I never met a single
emulator that could cope with this, apart from mine, although the
hardware terminals I'd encountered coped without blinking.
On 15/03/07, Dave McGuire <mcguire at neurotica.com> wrote:
On Mar 14, 2007, at 8:54 PM, Josh Dersch wrote:
Ahh, that answers that question. :) I'm about to hit the hay
right now, but I will see about reading some EPROMs tomorrow and will
let people know of their availability.
-Dave
--
Dave McGuire
Port Charlotte, FL
Actually,
yes, I was thinking along the lines of something which
actually made use of the original firmware. That quite possible
isn't practical though as presumably at least some vendors did
something really oddball rather than using an off the shelf CPU :)
But yes, "warts and all" through use of the original ROMs would
seem ideal to me (from an accurate as possible emulation point of
view). There's almost certainly far less of an issue with
publishing these ROMs for terminals I would have thought - lots of
the companies in question have either gone under or are hardly
likely to care (unlike the situation with MAME and games
manufacturers)
Along these lines -- does anyone have ROM images (preferably
program & character roms) for the firmware in, say, a VT100? This
discussion has piqued my interest in writing a hardware-level VT100
emulator (just what I need, another software project to distract me
from the dozen or so I haven't yet finished...). I own a VT100 but
I have no means to read the firmware off of it -- if anyone out
there has an image I'd love to play around with it. Any good
sources for hardware info on the VT100?