Trying to control vintage amplifier (TEAC A-H500) with Raspberry Pi - what's the communication protocol? 8-bit bus?

I'm trying to control my vintage amplifier from Raspberry Pi - I want to create a nice little Spotify streamer that on playback start will automatically turn on the amplifier and set the input selector(normally set to my mac). I have some success with it but I'm stuck on trying to figure out the protocol and wiring, thus my question. I'll start with writing up what I know so far.

I have succeeded controlling it through IR blaster wired to my Raspberry - it uses necx protocol and works well using gpio-ir device overlay. However it requires the blaster to be directed into IR receiver on the front plate of my amplifier and I'm not entirely happy with aesthetics. Putting it somewhere further in my room is also gimmicky. I'd like something more reliable and invisible.

The amplifier has 2 RCA remote control connectors on the back. I'd like to use them to communicate with my raspberry. I'm struggling to identify the protocol used there and therefore I'm asking here for help.

My amplifier is TEAC A-H500 - vintage device produced around 2000 year. It's a part of 4 separates that together create a hi-fi system. Remote connections are used to connect all 4 units together and control them from one remote control, using amplifier as signal receiver from it.

Originally I though that the RCA connectors use the same protocol as IR receiver, however after inspecting the schematics I think it's something else. I also tried wiring up IR receiver directly to these RCA but it didn't work. Here is a block diagram of internal wiring in the amplifier (complete schematics available in link above) We can see here that both RCA connectors are wired as IN/OUT and connected to designated pins in the microcontroller. On the right you can see internal IR receiver. Worth noting that in and out use single, shared wire. Minus on the RCA is grounded. Remote control buffer unit is a set of transistors (bus in and bus out pins are connected through them to RCA, top wire on the diagram):

So my understanding is that RCA connectors are wired to the microcontroller separately from the IR receiver and therefore they might use a different protocol than the one used in IR communication. The microcontroller used in the amplifier is ANAM1187A. Unfortunately I couldn't find any data sheets online. However the amplifier is a part of the whole system so I tried investigating schematics of other separates from the system. And bang! The compact disc player uses a different chip which I managed to find online. My reasoning is that since they are a part of the same hi-fi system and are meant to be connected I can lookup details of this chip as well. TEAC PD-H500 is the player. Details, schematics available in service manual
The player uses SONY CXP82616 microcontroller which datasheet is available online.

Ok so now we have much more details. As a first step I'd like to read the data from the amplifier. Once that is successful then I'll attempt sending some signals. Therefore my understanding is that I need to reproduce INPUT circuit from above microcontroller with my Raspberry Pi, in order to read signals sent from the amplifier.

Here's a shoot from the compact disc player schematics. We can see that the input is connected to the pin 2 and marked as RMCI

We take that and map it to the pin on the chip datasheet -> PE4/RMC

After this lengthy introduction I can ask my question finally. How to wire this up and setup Raspberry to communicate with the chipset? What kind of protocol is that? Is this UART? Or maybe it's actually the same as on the ir but I just don't know how to modulate it? I don't have that much knowledge / experience with hardware so any advice would be helpful.

Here's some copy&paste from microcontroller data sheet: (full data sheet linked earlier)

CMOS 8-bit Single Chip Microcomputer
Description
The CXP82612/82616 microcomputer is composed
of a CPU, ROM, RAM, and I/O ports. These chips
feature many other high-performance circuits in a
single-chip CMOS design, including an A/D converter,
serial interface, timer/counter, time-base timer,
fluorescent display controller/driver, remote control
This device also includes a power-on reset function
and sleep/stop functions which can be used to
achieve low power consumption.
Features
• Instruction set which supports a wide array of data types
— 213 types of instructions which include 16-bit calculations, multiplication and division arithmetic, and
boolean bit operations.
• Minimum instruction cycle 400ns for 10MHz, 122µs/for 32kHz operation
• On-chip ROM 12K bytes (CXP82612)
16K bytes (CXP82616)
• On-chip RAM 448 bytes (Including fluorescent display data area)
• Peripheral functions
— A/D converter 8-bit, 8-channel, successive approximation system
(conversion rate 32µs/10MHz)
— Serial interface On-chip 8-bit, 8-stage FIFO (1 to 8 bytes auto transfer),
1 circuit 2-channel
— Timers 8-bit timer
8-bit timer/counter
19-bit time base timer
32kHz timer/counter
— Fluorescent display controller/driver Maximum of 336 segments display available
1 to 16 digits dynamic display
Dimmer function
High voltage tolerance output (40V)
Hardware key scan function (Maximum of 8 × 16 key matrix available)
— Remote control receiver circuit On-chip 6 stage FIFO 8-bit pulse measurement counter
• Interrupts 13 factors, 13 vectors multi-interruption possible
• Standby mode Sleep/stop
• Package 80-pin plastic QFP
• Piggyback/evaluator CXP82600 80-pin ceramic QFP

• That looks like some sort of open collector bus, but unless you have an active transmitter to watch you may not deduce how it's used. Really this looks like a question for an audio user forum where you could try to find someone who knows about this gear. Such usage questions are off topic here, and it's not even really reverse engineering (itself on the border of not belonging) unless you have a known transmitter. – Chris Stratton May 30 at 17:08
• the user manual seems to imply that the wired remote control is not for controlling the amp ... it is for controlling the attached devices ... the amp passes IR remote signals to the devices through the RCA port – jsotola May 30 at 18:22
• However it requires the blaster to be directed into IR receiver on the front plate of my amplifier and I'm not entirely happy with aesthetics ... have you considered putting the emitter inside the amp, next to the sensor? ... internal reflection may allow the arrangement to work without actually pointing the emitter directly at the sensor .... other possibility may be to snake a light pipe (fiberoptic) from the back of the amp, through the amp, to the sensor ... that way you would not have wiring running through the amp enclosure – jsotola May 30 at 18:29

You could unsolder one of the remote control RCA connectors from the PCB, then replace it with a bulkhead RCA connector going to a 2-wire cable, then to an IR LED placed inside the amplifier right next to the amp's IR remote control receiver. Make sure the IR LED light shines on the IR receiver.

This would create an input port using the IR protocol, which you already know. Then simply feed IR protocol signal into the RCA, and it should work.

This should require no visible change to the amplifier.

• Could probably also just add to the IR detector output. Many are open collector, that's not clear here but could see how it responds to being grounded through a moderate resistor. If it's not open collector inserting an effective AND gate would do it. Of course the source would need to send only the envelope, without the 38 KHz or whatever. Of course the advantage of the LED is being an implicit optoisolator. – Chris Stratton May 30 at 17:15
• Thank you for your answer. I'm aware of that solution, however I don't want to solder anything inside the amplifier. It's quite a nice piece of vintage device and I want to keep it untouched. Nice idea tough! – xianoss May 30 at 17:34
• Yeah, that's a good reason, vintage gear needs love! (but sometimes it needs new capacitors too). How about sticking a plastic optical fiber through the hole in the existing RCA connector and place the "light exit" end of the fiber close to the receiver? – bobflux May 30 at 18:36