for the 'other systems' not all other systems are standard either it depends on the card or interface you have selected for use with them.... most dumb terminals, smart terminals, x86, *nix , s-100, cp/m, and tel systems use the standards....
most of the early home computer systems had their own 'standard' for serial systems and the add on devices varied from both.... you have to double check them to be sure of the interface and what is supported
For the Atari most devices are modeled after the 850 interfaces first port.... remember the 850's 4 ports differed in what they supported. The mio/pr connection/850 and other such interfaces used the Atari pin-out for the interface side of the cable... to be sure the Atari cable should be labeled as Atari 850 type cables and the intended purpose... Many third parties made such cables
Well, not to get too far off topic, but as for the other systems here is the type of interface:
TI-99/4a - PEB with and RS232 card
Apple IIe - Apple Super Serial card
C64 - Not 100% sure yet. There is an interface/modem repro I've been looking at though: http://www.ebay.com/itm/401033613594
And some others I've thought about:
Vic-20 - They made a modem for sure, but I'm not sure how useful it would be to get one online
TRS-80 Model 4 - Might be possible, not sure
Coleco ADAM - Not even sure if this is possible. I think they made an RS-232 card for it, but they're incredibly rare
Nothing stops you from wiring your own...I have rolled my own about 78% of the time, just make sure to follow the tiny numbers molded into the connector and the proper pin-out you need for each. The problem with rs 232 was each connector had either male or female ends and people mirror imaged the pin-outs and end up reversing things...
Is there a diagram that shows how to convert the Atari Modem Cable to an Atari Null Modem cable? Since the Atari one is different from the standard one, the diagrams I've found online won't do much good. You've posted a few ASCII diagrams in your posts, but I'm a bit confused.