Electronics > Beginners
RS232 Connection
Momchilo:
Hello,
I'm trying to connect my PC to a multimeter (Gossen Metratop 53) via a RS232 interface.
I'm using a generic COM port of my mainboard with an 1:1 pinout cable to the multimeter. The device has a female DB9 connector.
So there shouldn't be any problem with a direct connection without a null-modem cable or a crossover cable, right?
In the manual of the multimeter the pinout isn't mentioned (pdf manual).
I already tried the connections in the picture:
.
There isn't any response at all. The status of the input lines of the connection are seen in the attached picture.
If I'm using an USB-RS232 adapter cable, at least I can receive some data from the meter (IDN after every change in the interface option), but I can't send anything because the adapter outputs only TTL voltages.
Obviously the baud rate, parity, number of data bits and stop bits are for both devices the same (19200, N, 8, 1).
Do you have an idea what could be wrong? Because I'm quite sure I'm doing something wrong.
Thanks for your help.
Best regards
Momchilo
Nusa:
The connections you say you've already tried ARE crossover cables (or null-modems, the terms mean the same thing in this case).
Have you actually tried a direct connection, where pins 1-9 map directly to pins 1-9? That would be correct for a DCE-DTE connection.
ferdieCX:
Usually DTR --> DSR and DCD of the other device
See for example: https://www.lammertbies.nl/comm/info/rs-232-null-modem
You don't know the pinout of the multimeter, but you can check which pin is ground and then measure the other pins against it.
This way, at least you can guess which ones are inputs and which ones are outputs
Momchilo:
--- Quote from: Nusa on February 08, 2020, 11:37:36 pm ---The connections you say you've already tried ARE crossover cables (or null-modems, the terms mean the same thing in this case).
Have you actually tried a direct connection, where pins 1-9 map directly to pins 1-9? That would be correct for a DCE-DTE connection.
--- End quote ---
Sorry, yes. I have a RS232 cable with 1:1 pin mapping. I used this without anything between the PC and the meter. But then I can't get a connection.
After this I tried the other two mentioned methods without success.
After some more investigation there is something strange. I tried a simple echo test on the PC output with shorting Pin 2 and 3 at the end of the cable. No data were received. So I probed the pins with a scope. Pin 2 transmitted the data, should be pin 3. Then I shorted pin 2 with one pin after another and looked for receiving data. So I figured out pin 2 is TXD and pin 6 must be RXD. This is what I get directly at the COM port from the PC, because of the 1:1 mapping of the cable (measured).
Whats going on? I will investigate further, because this shouldn't be the case.
TK:
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?
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