|A bunch of serial cables? Maybe not... [source: author]|
Method one: visual inspection. DE-9 cables are usually either male-male or female-female at either end. If they're male-female then it's likely (but not definitely) an extension. All serial ports on devices are male, so it can be safely assumed that a female-female cable is some kind of serial or modem cable (there's more than one). It thus follows that, because CGA/EGA ports on PCs are female, it could be assumed that the corresponding cables are male-male. This is where it gets tricky though, so visual inspection is of limited use.
Method two: dismantlement. Yeah I probably made that word up but you know what I mean. One of the cables had removable covers at each end, so I took 'em off and had a butchers:
|A dismantled serial cable [source: author]|
|Wiring diagram of null modem cable [source: author]|
Method two: measurement. The next two cables could not be dismantled without destroying them, as they were factory-moulded. Time to get the multimeter out.
|Finding out which pin connects to which [source: author]|
|We have a match, but the pins are different numbers [source: author]|
|Wiring diagram of null modem cable with partial handshaking [source: author]|
Now the only issue I have is that one end of my cable is the wrong gender. I'm not buying some adaptor from a shop because I'm brassic. Instead, I'll take the dismantleable cable I've got and resolder it so it's also straight through. The only issue with that is that there are only 7 wires and I've got 9 pins to worry about. So now I need the CGA pinout to find out what's what:
|Pinout for CGA and EGA graphics standards [credit: what-when-how.com]|
There is one drawback. Although EGA uses DE-9 too (yes, DE-9), it needs those two extra pins (and hijacks the 6th pin, formerly used for intensity in MDA) in order to provide support for 2 bits per colour (hence two pins per colour). There are certain circumstances where the two can be interchanged but that would depend on compatibility between the monitor and the graphics card. If I wanted to maintain compatibility with MDA I would also need pin 7 but this will be exclusively CGA so I don't care.
Right, where's my soldering iron...
|Rewired null modem cable for 'straight through' operation [source: author]|
Took me a good 15 minutes to remember that there's a switch on the front panel of the 1084 where you can choose which input to display (like every other bloody monitor - what an idiot!). Anyway it's not like a rewired the cable for nothing - it still wouldn't have worked even with the monitor on the right input selection. So now it works like a charm. I think I'll do another article soon on the differences between CGA composite and RGB.