IBM PC5150 monitor wanted/question

From: Tony Duell <>
Date: Wed Feb 4 17:22:26 2004

> I have an IBM PC 5150 system unit and keyboard. I popped the cover and it
> has a Paradise Multi Video or somesuch card in it. The card has two DB9
> connectors, an RCA connector in the middle, and a little slide switch at the
> top.

This is _probably_ a combination MDA(or Hercules?) and CGA card, with the
slide switch selecting between them. One DE9 connectors (there is no
such thing as a DB9!) is probably for the monitor, the other may well be
for something like a bus mouse (!). The RCA socket is probably composite
NTSC out in CGA mode, and some totally non-standard thing in MDA mode (if
it's active at all).

There was a popular multi-I/O card in the UK (it came in Opus PCs, but it
turns up elswehre) that had that layout of connectors on the bracket.
There were also headers on the board for 1 or 2 serial ports, a parallel
port, floppy controller, and a joystick port (IIRC).

Now, how to be more certain...

Firstly, note that on MDA, CGA, and EGA, pin 1 of the DE9 is grounded.
That may be enough to identify which connector is for a monitor. On MDA
and CGA, pin 2 is also ground, but it's one of the colour signals on EGA.
So you can probably eliminate EGA fairly easily.

However, genuine IBM EGA cards (and some good clones) have a jumper which
connects pin 2 of the monitor connector either to ground or to the
appropriate colour signal. The IBM EGA card will drive an MDA or CGA
monitor (you don't get all the EGA modes, but you get more than an MDA or
CGA card would give you). You could check for this jumper, it's normally
near the DE9 conenctor.

If you've identified the monitor connector, then on MDA/CGA, and EGA
connectors, the syncs are pins 8 and 9. Power up the machine and connect
a frequency counter (or 'scope with a reasonably accurate timebase) to
each of those pins in turn. I normally use the LogicDart. Note down the
scan rates with the slide switch in each position (Power down the machine
before moving the switch, then power up again). Most likely one will
be TV rates (15570 Hz horizontal, 60Hz vertical), that's CGA. The other
will be somewhat faster horizotnally (around 18kHz?) -- that's MDA.

If you can get it working in CGA mode, try connecting the RCA socket to
the composite input on a TV or VCR to see if you get a picture.

> Here's the deal: I'd like someday to find an authentic IBM brand monitor for

Since you want an IBM brand monitor, it would appear you're trying to
make the machine 'stock IBM'. In which case you really want to get an IBM
video card too. THey're not that rare. And schematics exist should you
get a non-working one.

My first PC/XT (still running) had been 'upgraded' by the previous owner
who'd fitted that multi I/O card I mentioned earlier. Fortunately he'd
kept the original IBM MDA card. First thing I did was swap it back. I
then spent a long time tracking down an IBM CGA card to go alongside it.

No idea how easy these monitors are to find now. I have all of them (and
am not selling). The PC/XT I mentioned came with a 5151 MDA monitor (which
is now on my hacked PC/AT, and which I am using now). My first CGA
monitor was an NEC TTL RGB monitor which had been used with an Apple ][
(with a special card in slot 7). I modified the video circuitry of that
monitor (a few more resistors, and using the spare sections of some
buffer chips) to add the I signal.

Received on Wed Feb 04 2004 - 17:22:26 GMT

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