Philips/Magnavox LCD TV Service UART / ComPair

Question

I have a Magnavox 42MF438B/27 42" LCD TV. The IR sensor (or something related) quit working after awhile, so we have been unable to use the remote. Today, I was digging around behind the TV and noticed a "Serv U." port on the back with what looks to be a standard 1/8" jack. I figured this was a serial port for firmware upgrades and what not. Some quick googling revealed this assumption was correct... the manual calls it "Service UART". So I am assuming it is a standard 1/8" TRS plug, one ground, one send, one receive.

My hope is that I can send it commands... volume up/down, power on/off, etc. over this port. Trouble is, I can't find any specs.

Does anyone know what the voltage this serial port runs at? I'd rather not fry it with RS232 levels if it is just 3.3V. Then, any thoughts on what the pinout might be? Or, any safe way to find out?

Finally, any thoughts on where I can find protocol spec? I certainly have no problems poking around, but I'm hoping perhaps a Philips tech can help me out here a bit.

Thanks a ton!

Edit: I found another message board post where someone's manual says, "For use by authorized service personnel, for installers, integrators, do not connect RS232 without level shifter", so I think I am on the right track. 3.3V then I am guessing? I think I have an old USB->cell phone cable around with the appropriate adapter.

Edit #2: I found a service manual! It says their techs can connect with HyperTerminal... 38400 bps, 8/N/1. It also says I can access "Customer Service Mode (CSM)" by keying in the code 123654. For "Service Default Mode (SDM)", 062596menu or for DVB-T Service Mode, 062593menu. For "Service Alignment Mode (SAM)", 0625961+ (osd/status). That last character may be an "i", not sure. Program display option code, 062598menu. No specs on the port itself though, or what is in those modes. Still digging...

Edit #3: Seems the service cable adapter kit is called ComPair, or possibly ComPair II.

Answer 1

If you just need to shift the voltage levels, use a max232.

These chips are very dependable and resilient. I would suggest against building a logic shifting circuit.

Answer 2

Here is one circuit that will clamp ±15V RS-232 levels to 0-3.3V (or so) logic levels.

      1N4148
        |\     4.7k
Input --| >|--/\/\/\--+-------- Output
        |/            |
                      |  |
                   +-----+
                   | / \   3V zener diode
                    /___\  500mW
                      |
                     ---
                      -

This only handles the transmit voltages though, and won't amplify the logic levels for the receive (to the computer.) If you're lucky, your computer will recognise 3.3V as a logic high and 0V as a logic low. If not, you'll have to use a chip like a MAX232.

Be very careful with this. Make certain it works as required, use a multimeter and 12V supply before plugging it in to verify it. The worst thing you could do is damage the UART processor, which could be the main processor, which would brick your TV.