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I'm writing a new question after stumbling across this question which appeared to have similar symptoms: LG RS232 Commands partially working.

As the other question noted, the power on command ka 00 01 works, but power off ka 00 00 does not. I've tried several other common commands, and I can't seem to find a single one that works. Some useful information:

  • To minimize fault points, I'm using a Windows box, USB to Serial converter, null modem w/ straight through cable, and typing commands directly into PuTTY
  • I have two LG TVs and one of them works exactly as expected with the same hardware/software; I've confirmed I can turn it on and off and execute other common commands
  • The TV that does not work is a 42LH50; I've tried lots of troubleshooting including disconnecting everything from the TV except power, trying with SimpLink on/off, trying with KeyLock on/off, trying with password protection on/off, going into the Service Menu to ensure the serial port communicates at 9600 baud
  • The on command results in a reply as expected based on LG's RS-232 guide, but then the TV also returns undecipherable bytes that make no sense and do not follow the guide. The bytes do not appear to be the same each time and get written out through the duration of while the TV is turning on. After that, complete silence; no commands get a reply whatsoever (not even NG). I have the session log that I can post, but I don't have access to it at the moment. There are no problems repeating the experiment after turning off the TV by remote or power button.

Any help would be appreciated.

EDIT: Also putting this in the comments:

  • As far as I can tell, no command other than "power on" works. I spent a whole day troubleshooting my cables trying to send "power off" commands (since I read that "power on" commands don't always work (at least for USB) and believed that "power off" was the most basic command.

EDIT: Changed my title to be a question. EDIT2: Updated question title to better-reflect new information EDIT3: I discovered that all commands I tried reply with NG when the TV is turned off. The TV replies with OK with a command to turn on or off or get power status, so long as the TV is off.

Here is a sample 28 byte output in response to ka 00 01 ([] is space, ?? indicates a seemingly random byte:

Hex: |61 20 30 30|20 4f 4b 30|31 78 eb df|ff 18 f4 78|

ASCII: | a [] 0 0|[] O K 0| 1 x ?? ??|?? ?? ?? ??|

Hex: |fa 32 14 8a|ca c9 ff f4|4f fb ff 00|

ASCII: |?? ?? ?? ??|?? ?? ?? ??|?? ?? ?? ??|

The random bytes seem to be different each time, and a different number of them (I've seen 26 and 27).

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  • \$\begingroup\$ Is is specifically the 'off' command which doesn't work, or is it the second command (whatever it may be) which doesn't work? What happens if you turn the TV on 'manually' and then send an 'off' command? \$\endgroup\$
    – brhans
    Mar 1 at 19:56
  • \$\begingroup\$ what is your specific question? \$\endgroup\$
    – jsotola
    Mar 1 at 19:56
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    \$\begingroup\$ @brhans As far as I can tell, no command other than "power on" works. I spent a whole day troubleshooting my cables trying to send "power off" commands since I read that "power on" commands don't always work (at least for USB) and believed that "power off" was the most basic command, so no, it's not just the second command. \$\endgroup\$
    – tgpaul
    Mar 1 at 20:05
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    \$\begingroup\$ please make sure that only the power on command turns on the TV ... it may be a case of any command turns on the TV \$\endgroup\$
    – jsotola
    Mar 1 at 20:13
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    \$\begingroup\$ @tgpaul - sorry if I was rude, its just that I think most of us at EE.SE are tired of being tech support for poorly documented/designed consumer products. I know its frustrating to get it to work but most of us (I'd think) are not too keen on supporting crappy TVs that are put out in the world. I have upvoted your question, maybe with this rep you can comment on the previous question and see if any of the people there answer 3 years later. Good luck! (if you do get some resolution might be nice to post back an answer for future readers) \$\endgroup\$
    – Wesley Lee
    Mar 2 at 0:23

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