cctalk Digest, Vol 70, Issue 3

[Tony Duell]

> 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 

Was it? I can't think of many classic RS232 devices that have those pins 
wired. 

> > 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.

HP made one. Or more specifically HP made a 300 baud modem that plugged 
into their serial port and had no other power supply. Of course the HP 
serial port was a 50 pin microribbon connector with +5V, +12V and -12V 
power lines on it. Not test votlages, pins specifically designated as power 
outputs.

IIRC, just about all 50 pins were used for something, the full interface 
has both RS422 and RS232 signals on it. Very few HP devices implemented 
everything (the 98628 serial card for HP9000/200 machines comes close). 
But the power pins were almost always wired.

-tony