This article aims to document some of my findings while researching the inner workings of my D816 PABX. While several versions exist for all those models, the general operational principles are, more or less, the same.

Those units have a lot in common and the schematics I could find seem to confirm they're iterations of the same project, even though TA1232 has important differences, like not supporting digital proprietary phones.

The CPU is a classic 68k chip. My unit, particularly, uses a TMP68301AF-12 CPU. According to the datasheet, it's based in the 68HC000 core, produced by Toshiba. It's a 16-bit processor, with 24 addressing lines and 16-bit wide bus.

While the CPU might be different between versions (newer models do operate at 12 MHz, in contraposition to older models at 8 MHz), the pinout seems to be largely the same in all three versions.

KX-TD1232CE/1232BX etc.	KX-TD816E (UK)	KX-TA1232

A0-A23, as well as D0-D15 lanes are distributed all across the system, interconnecting several daughter boards and add-on cards. It seems like all additional hardware connects to this bus one way or another, and the IORN/IOWN lines (connected to the IC2, not the CPU) are to be found in all parts of the system. We have addressing, data and control lines, clearly establishing a system bus architecture.

So, the obvious conclusion is that this system is indeed a computer, and a m68k computer at it, albeit a so-called «embedded» one.

Devices seems to be connected to it in a similar fashion as ISA devices are typically connected to a computer: we have clock, interrupts and addressing to determine how the communication happens on the bus. How exactly is this communication happening is yet to be determined by further examination of the schematics and a healthy dose of reverse engineering with oscilloscopes or ESP8266's. I might update this article with future findings or move the entire thing to a GitHub repository, in the future.

As this investigation progresses, I might transfer this section to it's own article, since it's a bit of a rabbit hole.

The digital PABX's (KX-TD) got the KX-TD193 or TD193X as their standard Caller ID add-on card. This part features pretty standard FSK modems that output serial date for two serial-to-parallel converters, that then feed data to the system bus.

Funnily enough, there's a second card for KX-TD systems called KX-TD193DX, that also does DTMF (hence the extra «D» in the name, I guess), and this card also happens to be nearly identical to the TA-123293 card, which also had DTMF decoding facilities. The only significative difference I could spot was an extra IC, which communicated with TA1232 via four extra pins that KX-TD's don't have.