cctalk Digest, Vol 70, Issue 3

Jay West jwest at
Tue Jun 2 23:06:20 CDT 2009

It was written...
> Huh? The RS-232-port isn't supposed to deliver any power at all.
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.

> But I'd be surprised if a circa-70s modem was ever designed to use the 
> power from the RS-232 port to drive the modem itself. I'd expect it to 
> have an external power supply.
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 

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