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A question requiring a creative solution

Hello there :) I have been given a question which involves some thinking and understanding of the RS-232 standard but with an imaginary interface. I can use any equipment i want except a cable that connects my pc to a modem. The standard is called ABC, and it has the following pins: SG,RTS,DSR,DTR,RX,TX

i have to find which of each of the six pins making up the ABC interface actually correlates to the aforementioned pin functions. It is also mentioned that the terminal will not raise RTS before it receives DSR. Does anyone have any idea how i can find out which pin does, regardless of costs and equipment (except the DTE-DCE cable).

Thanks alot, i know it's an odd question but it would definetly help me see a creative answer to questions i wouldn't know how to tackle

Reply to
bar.eitan
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A homework question then.

I would think that if you start by reading up on the functions of each signal in the RS232 standard and imagine yourself with the simplest possible test equipment that would confirm the operation of the pins individually then you may be able to come up with your own solution. We will comment on it, if you post it up, and point out any ways of improving the method you come up with.

A good source of infomation, apart from the standards document itself, will be something like the "Hackers Handbook". They go through a number of ways of connecting up RS232 ports.

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Reply to
Paul E. Bennett

First, you've got to find the ground pin. I'd use a voltmeter to find a pair of pins with a 12V difference or thereabouts. This means that the negative end of the voltmeter is on one of the output pins (TX, RTS,DTR) and the positive is either on SG or on one of the inputs (RX, DSR). Keeping the negative end where it is, move the positive end around all the other pins and identify all pins where you get a 12V-ish difference (should be three in total). Now switch the meter to milliamps and measure the current between the negative pin and the three others you identified as giving a 12V difference. Only when the positive end of the meter is on SG, will you get a 10mA (or slightly less) current, since you've short circuited an output at -12V to GND. The RS232 drivers should have a 10mA short circuit current limit.

Now that you know which is GND and also which other two pins are inputs and which three are outputs, use a 12V source to connect one of the inputs to +12V. If one of the outputs also goes to 12V you've found DSR (input) and RTS (the output that went up - according to your story). If nothing happens with the first input, you've found RX, so try the other one (DSR) in order to find RTS.

How to find out which is TX and which is DTR? I have no idea. If you can make the equipment with the ABC interface send out characters, you can use a scope to see what's wiggling. Viktor

Reply to
Viktor

I'm not going to do your homework, but I'll give you a couple of clues. BTW, this is a great practical problem and one that you're liable to run into on some sad late night.

Clue #1: You can immediately sort the inputs and outputs with nothing more than a voltmeter. How would you do it?

Clue #2: What are the voltage levels for true and false RS-232 and what would you expect them to be for the corresponding output signals?

Reply to
Jim Stewart

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