NX-3300 Trying to build DIY programming/accessory cable

[LynxVortex]

Hi guys, I recently got a Kenwood NX-3300, and I’m working on a small personal project. I also have NX-800 and NX-820 radios, and I’m trying to build a custom cable/system to link them together and make them behave like a small network. The radio(nx-3300) programs are perfectly fine with the original KPG-36X cable, so I know the radio itself is okay
Now I’m trying to access the radio through the accessory connector directly, using jumper wires (TXD, RXD, GND) and a CP2102 USB-TTL adapter, but I get absolutely no communication. What makes it even more confusing is that I’m seeing weird voltage levels on the pins

TXD pin sits at about 5.49 V
RXD pin sits at about 3.19 V

So this clearly doesn’t look like a simple 3.3 V TTL UART. This makes me think that the KPG-36X cable is not just a basic USB-TTL cable, and that there is some extra electronics or some kind of enable/handshake going on. I’m not looking for software, I’m just trying to understand the hardware interface.
My questions:

Does anyone know what’s actually inside the KPG-36X? (I found one on the Internet, figure 8 in the photos. It looked like a simple UART and USB converter circuit to me.)
Is there some extra pin or signal needed to put the radio into programming/accessory mode?
Has anyone here ever built a DIY KPG-36X?
If yes, what was the trick that made it work?
Also, I’ve seen some third-party cables that seem to only use 3 pins and just a USB cable. How do those work?
Any info, measurements, or experience would be really appreciated. Thanks!

 

[kd4efm]

Mobile relay is only available on the nx5000 mobile. Best not to use the portable like that, and be careful, you could blow the usb chip and that's not warranty repairs if that happens.

The use of the TXD RXD like that is not like you think. Hence why mobile relay is only in the nx5000, and not the 3000.

 

[LynxVortex]

Mobile relay is only available on the nx5000 mobile. Best not to use the portable like that, and be careful, you could blow the usb chip and that's not warranty repairs if that happens.

The use of the TXD RXD like that is not like you think. Hence why mobile relay is only in the nx5000, and not the 3000.

Thanks! And I understand the warning.
I think there’s a small misunderstanding about what I’m trying to do.
I’m not trying to use the NX-3300 as a relay feature. I’m simply sending data to the radio and reading data back. This already works in my tests when I use the KPG-36X cable.
Now I just want to make my own cable that behaves the same way.
With the original cable, the PC detects it and the radio starts receiving commands. With a direct CP2102 USB-TTL connection to TXD/RXD/GND, nothing happens.
So it looks like the KPG-36X is doing more than just USB-to-UART.
Do you know what extra signal or condition is needed to make the radio enter that mode?