cctalk Digest, Vol 70, Issue 3

It was written... Not quite true, but a case can be made.... As per the spec, pins 9 and 10 are reserved specifically to provide test voltages. The current available is minimal and designed mainly for testing purposes with breakout boxes, etc. But it is there (from those that chose to implement it - which back in the day was most folks). Not only that, but it was not uncommon - in fact very common - to find lots of devices that got their power from DTR, etc. These devices had a lot of trouble with RS423, where the voltage was much less than RS232. I don't recall the exact numbers but RS232 was something about 12V swings and RS423 was about 3.5V swings. Other than that, the two were pretty much identical. People often connected RS423 to 232 devices with no problem at all. But those devices that expected power from DTR or pins 9 or 10 (on a DB25) had a bit of difficulty when plugged into a rs423 port. I can't imagine a 70's era modem getting power from the interface. Well, not a modem designed to interface to analog POTS lines. A line driver or short haul modem? Heck yeah, I've seen scads of them that got their power from the RS232 interface. Jay West