Panasonic KX-TA1232/KX-TD816/KX-TD1232 Hardware Architecture

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 KX-TD 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

This PABX is, in fact, a simple, embedded m68k computer. Memory and peripherals are addressed directly, there's no MMU. Also, albeit the system has a 16-bit wide bus, it looks like only the memory chips are using all those lines (see diagram below.) All other devices I saw were using only eight lines of the data bus. Addressing lines seems also to go under-used for most devices, e.g. expansion cards using A1-A5, or even the Caller ID card, using A1 and A5, ignoring all others.

Those systems seem to be typically equipped with four RAM and two ROM chips. TA1232's Service Manual seems to suggest they might not be always populated. On my TD816BX, they are. Since this is a purpose built embedded computer, rather than a general computer, we got SRAM chips soldered to exactly the number of lines they need, without room for larger chips.

I couldn't ascertain why the heck we have two small 32K chips as the first two SRAM IC's, although I find quite interesting the fact that the configuration dump does result in a file that's approximately 64 KB in size, and I remember reading somewhere that those systems hold the configuration in the SRAM's. How this is holding up after all those years scapes me, but the battery I thought to be specific to the RTC may have something to do with this.

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.