Monitor adapter

[CommodoreJohn]

So I've got a nice crisp PCjr monitor that I'd like to use with a CGA or EGA video card on a regular PC, and run an audio connector in to use the speaker, as well. Finding the pinouts is easy enough, but does anyone know what kind of pin header the PCjr connector is? If I could just buy a suitable male connector, I could wire up an adapter without having to chop up the cable...

[MattCarp]

Easy. The PCjr uses "Berg" connectors.

As all knowledge has been assimilated by Wikipedia, I provide you with this link:

http://en.wikipedia.org/wiki/Berg_connector

Looks like you should be able to find them from a Tyco distributor.

[CommodoreJohn]

So I discovered. I have suitable connectors on the way, and this page on pinouts.ru has pretty much all the information I need. The one thing I still need to know is this: what's with all the extra ground pins? Do they all need to be hooked into a ground, or is that just a a mark for unused pins? Is the ground provided by the monitor for the card, or for the monitor by the card? And since I want to hook an audio connector into the monitor's audio line, where should I hook the ground for that?

[Brutman]

I'll add some more information, which will probably raise more questions and doubts. ;-0

The tech ref for the machine shows two different types of grounds. There are multiple logic grounds and a shield ground. There are also pins marked 'Reserved' which by convention are usually not hooked up to anything.

You can (and probably should) tie the logic grounds to each other. I am not an electrical engineer, but you can (and probably should) tie the shield grounds to that as well. It doesn't make sense to have two different ground planes in the same machine.

The pins for the picture are all 'TTL' levels, which are a max of 5 volts. The audio output says '1 volt peak-to-peak biased at 0 volts', so it's weaker than the TTL signal levels used on the pins.

If I had to give a definitive answer, I'd check the connector on the PCjr side and I'll bet you'll find that all of the grounds are just tied together.


Mike

[MattCarp]

As we've wrestled with in the ISA adapter project, the additional ground signals act as a barrier to noise (crosstalk) on the signal cable. Instead of inducing noise on another signal cable, it induces noise on a ground wire, which conveniently shunts the current to ground. So we need those connected.

Shield and logic grounds are different. The logic ground is the reference point for the logic signal, so components need to share that to be able to interpret the signal. The shield ground should be a path to the earth ground, so that the shield protects the equipment from any induced noise, again, shunting the noise to the earth.

I haven't checked the schematic, but I'm not sure the grounds are tied together. If they are tied together, it would likely need to be near the actual ground of the power supply. One reason to not tie them together is, if there is some noise being picked up by the shield ground (say you're running the vacuum cleaner near your computer), you don't want that noise on the logic ground, or else the reference point for your logic signals will be corrupted! And then your computer would crash...

[Brutman]

I knew that post would perk your ears up ...

But I remember (maybe incorrectly) seeing the shield grounds tied nearby to the logic grounds. It's worth looking at the motherboard to see how they did it.

[MattCarp]

A quick look at the schematic in the tech ref and I couldn't see it.

I looked at the connector spec on 3-94 and 3-83 of the tech ref and it doesn't distinguish between a logic and shield ground.

I think it would probably be safe to tie them together.

[CommodoreJohn]

Interesting, but I'm a little lost now. For one thing, the pinouts.ru page only lists these as "ground," not as any of the specific types you mentioned; is there a chance you could post the official pinout? And, given a 9-pin D-sub connector for a CGA card, an RCA mono audio jack, and a Berg connector for the PCjr monitor, which should I tie to what?

[MattCarp]

Ok, from the Tech Ref, the "D" connector to 9-pin color display connector map is:

Code: Select all

A1-3 - not used
A4    - red           -  5
A5    - green       - 3
A6    - blue         - 6
A7    - intensity    - 4
A8-9 - not used
B1     - vertical     - 9
B2     - not used
B3     - horizontal   - 8
B4     - not used
B5     - ground       - 2
B6-8   -not used
B9      - ground      - 1

From the back of the PCjr, looking at the connector, it goes A9 to A1 from left to right on the top row, and B9 to B1 from left to right on the bottom row.

The DB-9 connector pin numbering is 1 to 5 on the top (5 pin) row, and 6 to 9 on the bottom.

In the tech ref no distinction made about the grounds. If there is a shield to the cable, just ground it at one end, the computer end.

-Matt

-Matt

[CommodoreJohn]

Huh. So if the other ground pins aren't important, I should be able to just hook in the ground-shield on the audio connector to the CGA ground, right?