[TRLog] Need info for self-powered serial interface

[Tom Hammond NØSS]

Hi folks:

Sorry if this is off-topic, but I'm not sure there's a better venue. If 
there is, please let me know.

I'm working on a design of a PC board to supply an RS-232 interface between 
KENWOOD transceivers and the PCs serial port.

Note that I'm NOT BUILDING RS-232 interfaces (I'll leave that to W1WEF and 
others)/ I'm only working to design a PC board for others to use when THEY 
construct the interface. Once completed, the info will be available to 
ANYONE who might wish to use it. I might even make a limited run of PC 
boards available (once proven that they work) at a quite reasonable price.

I'm well aware of the 'economy' interfaces where TTL levels are used. 
However, I want true (or at least 'truer') RS-232 levels, AND I want opto 
isolation of the rig from the PC.

I'm using a MAX202 RS232-to-TTL convertor and an SP2501-4 quad opto 
transistor. I have the basic design completed, BUT I need some info on 
where/how to steal the DC directly from the serial port.

I now that Kenwood (apparently) uses an external power supply for the 
interface, but I'd like to have mine powered from the serial port.

One 3rd party vendor appears to use steering diodes from serial port pins 
20 (DTR) and 4 (RTS) & 5 (CTS) (together) to obtain power for his Kenwood 
interface. But the Kenwood schematic appears to imply that, on their 
interface,  pins 5 (CTS) & 6 (DSR) are tied together, pin 4 (RTS) is 
separate, and pin 20 DTR) is not used.

So, I'm in a quandary with regard to WHICH pins I can safely use to steal 
+5VDC at a few mA for powering the MAX202.

Please reply DIRECTLY to me, since I'm sure there are others who are NOT in 
the least interested in this possibly off-topic thread. If anyone IS 
interested in the thread, please drop me a note and I'll CC: you with any 
replies.

Thanks,

Tom Hammond   N0SS


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