This is my attempt to reverse-engineer the LDG AT-1000ProII Meter protocol in order to eventually implement software control AND meter reading. It's very much a work in progress, so no judging.
To communicate with the tuner, I needed a USB to TTL virtual serial adapter. The TTL is important, the voltages are different than for RS-232. Fortunately, the USB adapter I acquired takes care of all the level shifting for me. If you want to use an RS-232 port, you need to make or buy a level shifter to convert from 12V RS-232 to 5V TTL. A suitable item can be found on Amazon if you search for "FTDI USB TTL Adapter."
The other thing you need is a 4 PIN mini-DIN connector. These are terribly hard to find, but mouser has one here. Note that S-Video cables, while having the same connector, cannot be used because they bridge some of the pins together. In fact, you should NEVER use an S-Video cable with your tuner, becasue one of the pins on the tuner is +12V and you don't want that getting fed back into someplace it doesn't belong. Also, do NOT get the 4-pin connector from Amazon - they tend to melt.
LDG published a document called "meter protocol" for the LDG M-1000, which, due to it seeming to suddenly want to vanish from online, I have included in my repository. Much of my information for this project came from there.
Besides the ones documented in the Meter Protocol document, I discovered 2 additional important commands:
S - places the tuner into "meter mode," which enables it to send the meter telemetry described in the next section.
X - places the tuner back into "control mode," which turns off meter telemetry.
[blank space] - wake up the tuner before sending it any commands.
Placing the tuner in "control mode" prior to issuing a command makes it much easier to parse command responses that come back from the tuner.
These commands don't have any responses, althought the "wake" command does cause a bunch of meter telemetry to be sent when in meter mode.
The data that comes from the LDG Tuner as intended for the M-1000 meter follows a certain format that looks kind of like this:
2 bytes representing forward power
2 bytes representing reflected power
2 bytes of something I can't discern, but it varies by band / frequency
2 byte End Of Message marker, always 0x3b3b or ";;"
Note that all values come across in Network Byte Order, so if you're on a little-endian machine like me, you have to use htons() to convert them before doing any additional math.
The forward and reflected power can be approximately converted to watts using the formula found in tuner.cpp. VSWR is then calculated based on the values of forward and reflected power.
To send any command and have the tuner parse it properly, I found that you need to send a wake command first, followed by a 1ms delay before sending the actual desired command. Additionally, if the tuner is in meter mode, it is a good idea to send a "control mode" command first before sending the desired command. Examples of this can be found in the source code.
I hacked together a Windows version in C# .Net. It's up here on Github somewhere and a lot more feature-complete than this. Serial port programming in .NET kinda sucks, tho.