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Power over RS-232 idea

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oktology

Experienced Member
Joined
Mar 24, 2023
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Houston TX
I've noticed that the RS-232 pinout for the DB-25 connector has two Reserved pins and one unassigned. I've seen a DB-25 pinout that I can no longer find that uses two of those pins to supply 12V power (I want to say it was an X.25 pinout, but I'm not 100%), and I was thinking this could be really useful for powering things like the various WiFi modems.

My question is this: is anyone aware of any RS-232-speaking device that uses pins 9-11 that could cause the magic smoke to escape if fed 12VDC on those lines?
 
Should not be an issue. On DB-25, pins beyond 8 were not used, except 20+22 for DTE stuff. This is why they could use a DB-9 later.
 
Well, the V.24 spec calls for pins 9 and 10 to be used on DCE for a power indicator, not as a power supply. So if you decide to use these pins as a supply, make sure that they're current-limited. Synchronous modems, of course had secondary channels and used additional pins. It's important to understand that the 9 pin connection was for use with async communications. Read about it here: https://www.arcelect.com/rs232.htm
 
V.24 is not RS232. DB-25 was also used by Apple for SCSI - but it's completely irrelevant what that has on these pins, as the question is about RS232.
 
pin 9= +12v pin 10= -12v
I haven't seen anything in decades implement that.
Thats no guarantee you won't see magic smoke.

joe
 
Chuck(G) said:
Well, the V.24 spec calls for pins 9 and 10 to be used on DCE for a power indicator, not as a power supply.
Click to expand...
V.24 it was! Thanks for this.

jlang said:
pin 9= +12v pin 10= -12v
Click to expand...
That's how they're assigned for V.24, so that was my thought.

I think I'll go ahead and add +DC on pin 9 to one of my portables and wire an adapter for the WiModem that draws power from that pin. No reason it couldn't provide 5V instead of 12V, the M102 has an internal 5V supply rail, and the WiModem is normally USB powered.
 
Timo W. said:
V.24 is not RS232. DB-25 was also used by Apple for SCSI - but it's completely irrelevant what that has on these pins, as the question is about RS232.
Click to expand...
RS232 - V.24/V.28 - IS2110 - X.20 bis (for Async) -
X.21 bis (for Sync)
Click to expand...
It is RS232: https://www.electronics-notes.com/a...-communications/rs232-v24-basics-tutorial.php
I can cite the ITU spec as well. Clearly you have never experienced a Bell 408 modem.

It was even applicable to the PC: https://www.minuszerodegrees.net/oa/OA - IBM Binary Synchronous Communications Adapter.pdf
 
RS449 is a mess; I thought that RS530 largely supplanted it. I suppose most industrial equipment now uses Ethernet-derived protocols (UTP, fiber...). RS422 saw quite a bit of use in non-communication applications (e.g. hard disk controllers).
 
oktology said:
I've noticed that the RS-232 pinout for the DB-25 connector has two Reserved pins and one unassigned. I've seen a DB-25 pinout that I can no longer find that uses two of those pins to supply 12V power (I want to say it was an X.25 pinout, but I'm not 100%), and I was thinking this could be really useful for powering things like the various WiFi modems.

My question is this: is anyone aware of any RS-232-speaking device that uses pins 9-11 that could cause the magic smoke to escape if fed 12VDC on those lines?
Click to expand...
RS-232 specified the allowable voltage range for all inputs and outputs. Your +12v and -12v are well within that range. The specification also indicated that the output drivers could be connected to any voltage from -25V to +25V without permanent damage. A device connected to your modified pinout system may not work properly, but you shouldn't let any of the magic smoke escape.
 
It's not the voltage that matters, but rather the current +12V at 50A may well fall within the voltage specs for RS232 signalling, but not the maximum current. I believe the spec says that the load should be from 3K to 7K, but don't quote me on that.

In any case, I believe the spec says that any signal must be able to survive an indefinite short circuit. So put that into your work.
 
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