Re: Figuring out a serial port

Date: Mon, 09 Sep 2013 11:23:08 +0200
Message-ID: <20412177.6301378718588803.JavaMail.www@wwinf3706>
I agree with SD. Tracing the hardware circuit out from the connector is probably going to be your best bet.

This will tell you which pins are not connected and (more importantly) which ones are connected to ground or power. You should also be 
able to find this out with a multimeter set to a low continuity range. You can then avoid any power pins!

If you are lucky, the hardware may have a UART chip inside which will mean that you can track down the data sheet and work out the circuit 
both forward from the UART and backwards from the connector. If not, you will just have to 'wing it' and trace out the circuitry the hard 

Be warned, the serial levels may not be RS232. They may be logic level, 20 mA current loop or (even older) 60 mA current loop. Connecting 
directly to an RS232 port could spell disaster if the equipment doesn't actually have this type of input/output stage.

Any chance of posting a photograph of the insides? This may elicit some further suggestions.


> Message Received: Sep 09 2013, 03:25 AM
> From:
> To: "" <>
> Cc: 
> Subject: Re: Figuring out a serial port 
> -- 
> Sent from mobile phone (so please have understanding).
> On 8 wrz 2013, at 21:44, Justin <> wrote:
> > Is there a trick for figuring out which pin is which on a serial port that can be executed with a multimeter and an ancient no-logging 
Heathkit oscilloscope?
> If this is your only testing equipment then it may be in fact tricky.. 
> First, have you checked the levels? Is it really an RS232 with its notorious logic levels? Or something else? This you should be able to 
check with your scope. 
> Next I'd of course connect a logic analyser with or w/o level converters, depending on the outcome of the previous test. But since I 
understand that you have none, the "get one" suggestion is quite appropriate IMHO :-)
> How about looking closer at the hardware itself then? I still remember very well the lesson Ruud once gave me by deducting virtually 
everything (including error in the silkscreen!) about an unknown to him piece of hardware by merely looking at an (unpopulated!!) PCB of 
some seven ICs of various sizes. After this experience I know that a lot of information can be gained this way. At least you should be 
able to eliminate some pins from the equations. 
> -- 
> SD!
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Received on 2013-09-09 10:00:52

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