Electronics > Beginners
RS232 Connection
Nusa:
On the PC, are you going from an existing back panel connector, or from the IDC-10 connector on the motherboard? If the latter, I'd dig out your motherboard manual (on-line if necessary) and check the pin-out for that connector. Not everyone used the same standard, sadly.
rstofer:
There is a problem, from time to time, with the concept of TX and RX. You would think that TX always transmits but some equipment means "hey, I want to connect my TX (which will receive) to your TX". It's truly a PITA.
The other problem is voltage levels. If you are transmitting at TTL levels (0-5V) you can't interface with RS232 which could be +12 .. -12V and, worse, it's upside down.
You have the right idea, check for which pin transmits on your PC and which pin transmits on your DMM. I would ignore the other leads, it is doubtful tht they are even used. I would get pins 2,3 and 5 straightened out first (DB9 pinout). I don't know what to say about you voltage levels.
https://www.stratusengineering.com/rs232-9-pin-pinout/
Measure the actual voltage swings before deciding you need to invert. Ordinarily, RS232 idles low while TTL idles high
https://www.sparkfun.com/tutorials/215
Once you find out how the voltages shake out and you find out how to connect pins 2 & 3 on both ends, you will know whether you need a crossover or straight cable.
This serial stuff is always a challenge.
Momchilo:
--- Quote from: TK on February 09, 2020, 12:46:15 am ---On a DCE (PC), TX is pin 2. Pin 6 does not make sense for RX... How are you counting the pin numbers on the DE9 connector?
--- End quote ---
I counted like this (male - right side):
For the measurements I simply used the printed numbers next to each pin in the connector to not make any counting mistake.
But I figured out the problem. The stupid connector between the mainboard and the PCI-slot in the PC is different/wrong.
You can see the pinout in the attached pic. So it's not RS232 compatible, I don't know what this is.
Nusa:
RS-232 doesn't specify connector pin-outs, just the signals.
And that is one of the two most common header pinouts for serial ports. You've just got the wrong add-on cable.
What you currently have is like this: https://www.pccables.com/Products/07121.html Can also be found with a ribbon-style DB9.
What you need to buy or make is this: https://www.pccables.com/Products/07120.html
That site has good data and pictures of how it's assembled. You can buy from wherever. Include "Everex" as a key word if you search for other sources.
If the one you have has a solder-type DB9 on one end, and you can do basic soldering, you can rework it yourself to be the other configuration.
Momchilo:
--- Quote from: Nusa on February 09, 2020, 07:05:07 am ---If the one you have has a solder-type DB9 on one end, and you can do basic soldering, you can rework it yourself to be the other configuration.
--- End quote ---
After I noticed the different pinout I resoldered the wires on the DB9 connector and now everything works fine. I didn't think there could be any difference.
Many thanks to all of you.
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