TRS-80 Plug 'n Power Controller
The original TRS-80 Plug 'n Power controller (catalog 26-1182)[1] was Radio Shack's 1981 answer to home automation. "Plug 'n Power" was actually the Radio Shack branding for products that speak the X10 protocol. If you're not familiar, X10 uses modules that send signals through the house AC lines to turn on and off lamps and appliances. They also made special wall switches and outlets so you could integrate X10 directly into the walls of your house, if you so chose. I say chose, but actually X10 devices are still made and sold, and the X10 system has been extended to control devices via IR, radio, and added sensors like motion detectors, cameras, and windows switches to make an entire security system.
However, we're only concerned with the original X10 power line system in this context. The TRS-80 plug n power system came with a small gray box that plugged into the cassette port and and AC outlet. You could then plug your tape recorder into the box, and by flipping the switch on the top, pass the signals through so you could either use the box or your tape recorder without having to unplug anything. The unit comes with two cassettes, containing a machine language "Home Control" program, for the model 1 and 3, and the color computer.
However, what my unit didn't come with is any kind of manual, even though it's in the original box. And neither do any of the other units that are available on eBay, which is where I got mine. As it turns out, I was very lucky to get the cassettes! There is next to no information on this piece of hardware anywhere online, besides the fact that it existed. That means no schematics, no software, and no programming information.
Naturally, the first thing I did was back up the content of those cassettes. There are instructions on the tape labels themselves that show how to load the tape with SYSTEM. To my relief, both the 16K and 32K versions of the program loaded just fine on my Model 1 without issue. I used the cmdfile</cmd> utility under LDOS to back up the programs into CMD files. I also digitized the tapes using a tape recorder, although I am still unable to read them using any of the wav2cas program's I've found. Even so, I can always take a snapshot of ram with the programs loaded using the TRS80EI2, and you should expect a full backup soon.
The next part of the problem was trying to figure out how to use the software. The program it comes with has two modes, a direct control mode and one where you can set up a schedule. The direct control method was pretty simple to figure out, but the scheduling proved to use some weird unintuitive keyboard shortcuts. Lucky for me, I found an old article from Creative Computing
[2], where, in 1982, somebody wrote a review of this unit, and helpfully included how to operate the software (thanks again to Jason Scott who has collected and uploaded thousands of old computer artifacts to the Internet Archive over the years).
But, there was still a nagging question. On the box, it said that there was a way to control the X10 modules from within a BASIC program. No doubt that this information would have been listed in the manual, which doesn't exist. The obvious way forward would be to reverse engineer the hardware and software.
Reverse Engineer
At first, I thought that the little grey box might get a command sent to it from the TRS-80 and then get sent out on its own time, but this idea was quickly abandoned when I opened the box and found only a small transformer and a PCB with just one quad op-amp chip. There were no smarts in here; obviously the TRS-80 must be doing all the work. I know that the box must be sending some kind of clock signal to the TRS-80, because the included control program came with an on-screen clock, which would not update unless the box was connected. The expansion interface also has a clock, but this accessory does not require it to work. The first step was to determine what kind of signal the box was creating for the TRS-80. I put a scope on what would be the tape out / trs80 in pins, and found that the box sends out a pulse about every 8 milliseconds. This is roughly 120Hz. The X10 protocol sends out little blips on the power line at the zero-crossings of the AC power, and that's really all there is to it; sometimes it sends out a 1ms blip of 120Khz at the zero crossing (what I will call a Mark), and other times there is nothing (a Space). This pulse signal must be the zero pulses; it looks like the trs-80 just sends out a pulse at the zero crossing when required.
The next step was to disassemble the included machine language program, which left me with a very confusing ASM file. It's easier to read than a hex file, but still very hard to read without any comments, especially because I am new to assembly (coming from higher level programming like C). However, I was able to find the section of code that interacts with the grey box. The standard TRS-80 uses only one of the 256 z80 ports (it memory maps everything else), and that port is at 0xff for the cassette. So, by searching the disassembly for any IN or OUT opcodes, I quickly found the sections that deal with the box interface, specifically, the section that would wait for a zero-crossing and send out a Mark. I found the section for Space too, but I couuldn't get the code to work in my assembler programs, so I duplicated the Mark code and changed it so that it wouldn't actually send out anything, but would still stop anything else from happening until that zero crossing was received. While looking around the Mark code, I also found a section that would check the TRS-80 ROM to see if it was a model I or model 3. The model 3 has a faster system clock, so the program would need to be slowed down or the Mark would be sent out at the wrong time. Since I only have a Model 1, I omitted this from my assembly program.
With these fundamentals established, I can build up the rest of the building blocks of the X10 protocol. There's plenty of information online[3] on the details of X10. For a basic explanation, each X10 messages starts with three Marks and a Space. Then, 9 bits are sent, which contain the House code and either a Unit number, which selects that unit for further communication, or a Command, which makes the unit turn off or on.