12 Jul
2005
12 Jul
'05
2:33 a.m.
On Mon, 11 Jul 2005 23:14:15 +0100 (BST)
ard at p850ug1.demon.co.uk (Tony Duell) wrote:
Its my understanding (but I've only seen it as a rumor, not as hard
technical fact) that the reason the original IBM Monitors plug into the
PC Power Supply and not directly into an outlet is that they were
vulnerable to damage (probably the same horizontal drive problem) if
left powered on independent of the PC.
I at one point had an original IBM monitor that had a direct power cord
connector and understood at the time that it was one of the very early
monitors for this reason. (I also at one point had an IBM PC-1, the
first-generation IBM PC, detectable from later versions by it's four
rows of 16K chips and the powersupply and I/O channel backplates being
painted black. I grow angry just thinking about the row with the
landlord that led to me giving that machine up)
The scanning frequencies of the EGA and MDA cards should be real
close (MDA outputs 348 lines of res, the EGA 350). You shouldn't
have a problem there (I won't comment about the jumpering/shorting
though), but keep in mind the original IBM mono monitor was very
very touchy. If you plugged it into a CGA card, it was known
to...ummm...explode...
That's going a bit far. It's very easy to burn out the horizontal
output transistor and maybe associated components, but it won't
explode.
There is no horizontal osciallator in the MDA (5151) monitor. The
Hsync line goes to the base of the horizontal driver transistor, the
collector of which is transformer-coupled to the base of the
horizontal output transistor. You can therefore attempt to drive the
horizontal system at just about any frequency, but if the horizontal
output stage is driven way off resonance (e.g. using the CGA
frequncies), it will do nasty things.
I won't swear to it, but I had thought there was a way of
configuring
it specifically for a mono monitor. Don't hold me to it though.
There is. There are switch settings for the EGA card to drive the MDA
monitor, CGA monitor, or EGA monitor (the latter can operate at 2
different horizontal frequencies). The reason I mentioned the jumper
link is that a lot of people forget to set that correctly when linking
to a CGA or MDA monitor.
-tony