Converting Serial Cable for UNI-T UT61E to USB

A while ago, I bought a UNI-T UT61E  to replace my 20 year old DMM, so I was very pleased about the new features compared to my old meter. The cable supplied with the multimeter is only RS232C and because neither my desktop nor laptop have such a port, no further investigation was done.

While cleaning up my desk, I found the software CD of the UT61E and it reminded my of the lacking USB connection for my DMM. So I checked to see if this could be upgraded to USB.

Sure enough I found this article and it looked simple enough to make the conversion myself.
Well, after tinkering around and trying to implement the same schematic, I couldn't make it work.
Not with a CH340 nor with 2 other models of PL2303 USB-serial converters. No matter what I did, the serial monitor would produce only garbage.

So, I decided to give it another go and go back to basics. I checked around what I had lying around in my stock and found a couple of DIL and SMD packages of 74HC02 and 74HC04.

Schematic using logic gates

I first mounted everything on a breadbord using the DIL package and experimented with the correct resistance R1. It turned out to work just fine with 1kΩ. The header CON1 is connected to the corresponding pins of the USB-Serial converter (CH340 or equivalent). It works both on 3.3v and 5v.

Next step was to depopulate the original pcb inside of the IR-receivr. I only kept the IR-diode.

 

I then put some kapton tape on the backside of the board and soldered everything as tested on the breadboard. I replaced the standard through hole components by SMD components. I reused some solder pads on the PCB as mount points for my cable. This also fixated the floating HC04 chip.
I kept 3 of the 5 wires. Brown = GND, White = VCC, Yellow = RX (actually TX, but it connects on the RX pin of the USB-serial converter).

 

After careful mounting and sealing of the IR-receiver, I was able to slide it back into place. As you can see, the setup with logic gates only takes minimal space inside the enclosure.

Because the UNI-T UT61E software only recognizes comports 1-4, I installed my CH340 USB-Serial converter on COM3. It works like a charm