RS-232 Fix

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RS-232 Fix

Postby RichLeary » Jan Tue 15, 2008 2:04 pm

My Micro-KIM is working great but the RS-232 interface (transmit only) does not work with all machines. I found that for some I needed to bias the static voltage on pin 2 to a true negative value. The simplest approach I had success with was just to use a 9v battery with the positive terminal connected to pin 5 (ground) of the DB-9 connector and the negative terminal connected through a 1000 ohm resistor to pin 2.

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Postby vbriel » Jan Wed 16, 2008 10:55 pm

That seems like a possible fix, although I'm not sure of the side effects from doing this. The gnd voltage vs negative voltage was used because of not having the room to create a dc-dc converter circuit.

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Postby RichLeary » Jan Thu 17, 2008 8:37 am

With the 1000 ohm resistor and the existing 220 ohm resistor in series with the Q7 output there should be no ill effects on Q7. The -9v through the 1000 ohm resistor does limit the positive swing to probably +3v or so. I did not put the scope on it to check that but +3v would still meet RS232 spec and a little less even would still work and does. I made the modification as a totally external adapter with two DB9 connectors and the 9v battery connector. The Micro-KIM itself is not modified at all and if I don't need the adapter for a specific machine I just unplug it.

One good solution to the negative voltasge issue is something like the MAX232 family of RS232 drivers that have their own built in charge pump.

I looked at using something like the COMPSys A232DB but the existing Micro-KIM would have to be modified to bring true TTL level non-inverted input and output signals out and I did not want to do that.
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Postby mfortuna » Jan Sun 20, 2008 9:37 am

There's another way of doing this but it does involve more mods. You can use a handshake signal (I forget which one, It may be DTR) to get a negative voltage on TXD. This is a trick parallax used on their stamp to RS232 converter.

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