trying to connect MP730424 DMM with rs232 but get Timeout error

I don't have this instrument, but here are some things to try.

Perhaps the transmit and receive pins need swapping. Sometimes instruments require a cable (e.g. null modem cable) to attach to the usb-to-RS232 adapter.  Worth trying both straight and swapped.

Also, presume the USB-to-RS232 adapter is functioning? You can check by unplugging from the instrument and shorting the transmit and receive pins and testing with serial terminal software (such as PuTTY).

Also, might be worth checking the instrument config menu, in case the default isn't 115200 (user manuals can be incorrect).

You should try all combinations of CR/LF and see which one responds to your *IDN? But make sure to power-cycle the instrument before each attempt, because an incorrect line termination leaves instrument software in a messed up state often.

Also, you could try different PC software (to eliminate potential PC software issues), for instance pyvisa is good. Or just test with serial console software.

It looks like an Owon XDM2041. The same (poor) documentation is on the web if you hunt for it. This discussion on eevblog (several message in) suggests that it will talk easily via the serial port.. The poster makes no mention of special cables so I think that you may have a basic comms issue.

Try connecting to it with Hyperterminal or your favourite terminal programme and send messages by hand. 

https://www.eevblog.com/forum/testgear/owon-xdm1041-the-unknown-multimeter/25/

MK

Most devices will need '\n' for the EOL/termination character. If not, try '\r'.

- Gough

Hi Shabaz, thanks a lot for the suggestions. 

- null modem: I now tried this but didn't help

- usb-to-rs232: I tried shorting the read and write pins and it works. tested with NI Max. 

- instrument configs: I checked and they are 115200, None, stop bits 1, data bits 8. I have checked that the cables and the serial software are all using the same. 

- I tried *IDN?\r, *IDN?\r\n, *IDN?\n, *IDN?. all gives a timeout error.

- PC software: tried with NI Max and with pyvisa. 

could the instrument be defective? how to test? 

maybe hardware flow control on RS232 is required? As usb-uart adapters are often only tx/rx/gnd, so maybe a computer with hardware uart and a proper cable would be necessary for a test... (EDIT - if this is logic level RS232 [despite the fact that photo suggests that it contains Sipex SP202 level converter], connecting to the standard RS232 can damage it)  from the photo below (if it is the same PCB as in MP730424) there is only one TX and one RX on SP202 used, so no hardware flow control

or cable pinout? in the times of old, (serial) cable pinouts for various equipment were really important - cable for one manufacturer was usually incompatible with equipment of different brand...

BTW. below there is a discussion about this DMM, but nothing about RS232

community.element14.com/.../software-options-or-interface-specs-for-the-multicomp-pro-bench-dmm

Which USB to RS232 adapter are you using - if the DMM uses true RS232 it may very well not work with a logic level adapter.

Do you have a scope (ideally with RS232 decoding) - if so then connect it to monitor the RS232 RX and TX pins.

Found some more info here:

https://github.com/PetteriAimonen/owon-xdm2041-info/tree/main/mcu_board

This would suggest that the DMM is  a logic level RS232 so it won't work (reliably) with a a true RS232 adapter on the PC.

You can check this - disconnect the lead from the DMM and use another meter to measure the voltage on pins 2 and 3 relative to pin 5.

If all the voltages you see are between 0 and +5V it is a logic level interface.

If at least one of the voltages is more than 3V  -ve  it is a true RS232  interface,

If both voltages are less than +/-0.5V is not a functional interface.

There are some more tests that can be done with a meter to check that you have RX and TX  correctly connected to their complements on the pC lead but we'll leave that until we know if we have the right voltage levels.

MK

michaelkellett said:

This would suggest that the DMM is  a logic level RS232 so it won't work (reliably) with a a true RS232 adapter on the PC.

Looks like there is a Sipex SP232 RS232 line driver/receiver on the circuit board.

Where did you get the picture ?

MK

int the repo you have found there are also photos of PCB....
https://raw.githubusercontent.com/PetteriAimonen/owon-xdm2041-info/main/mcu_board/mcu_board_top.jpg

From the link you provided above.