Panasonic KX-TD816BX Hardware Insights

This system got no more than a few hundred KB's of RAM. System configuration is stored on a EEPROM (I mean… is it? Couldn't confirm). The CPU is a m68k chip (yeah, same as Macintosh computers and several embedded devices of the era). This unity got the TMP68301AF-12 variant of that chip, manufactured by Toshiba.

The ROM complex is detachable, for reasons I deem not completely clear, since those chips are already seated on sockets. See below for more information on that detachable board.

As mentioned below, this unit has P401 software, dating back to 1996. It's an old ROM. The system seems to not have more than 320 KB of RAM. By all hardware measures, it seems to map to at least version “-1”, which also corroborated by this and this posts:

KX-TD816 Chip information:
-0 P011 Non-DPITS pre-DPITS 8MHz CPU. 2MB, RAM 256K
-0,5 P051I / first DPITS 8MHz CPU. 4MB, RAM 256K
-1 P401A / B DPITS 8MHz CPU. 4MB, RAM 256K (Suffix of Serial No. "N?"), RAM 1M (Suffix of Serial No. "O?")
-2 P301C / H 12MHz CPU. 8MB
-3 P301I / O / U 12MHz CPU. 8MB
-4 P311D / H
-5 P311K / M / P
-6 P311Q
-7 P311W

And this webpage also lists a fuckton of other versions.

In addition to those sources, this document seems to confirm we got at least a -1 unit. I used to believe I had -4 software level because I was using a programming guide that seemed to be 1:1 with my ROM, but, in fact, I confirmed later that several newer features were missing.

Critically, we're missing support for newer expansion cards, like the KX-TD193DX Caller ID card I've bought. Other than that, the losses are a bit minor. There are some features in that document that I kind of wanted, but none I can't live without (FFS, I could've even live without the central itself, so everything I got thus far is already pretty dope.)

As for the Caller ID card, this is an adventure on it's own, and this KX-TD193DX card is basically the same card (TA123293) used by TA1232, the non-DPITS, older version of TD1232, from what I gather. TA1232 itself is a eerily similar project. I'm discussing those topics on a different article.

According to information from this document, TD816 -1 units cannot be upgraded to level two. I assume that's due to the 8 MHz CPU being soldered to the motherboard, since the same upgrade is possible on TD1232. Unless TMP68301AF-12 really turns to be a 12 MHz (and operating at 12 MHz), we're dead in the tracks here.

If the CPU is really a 12 MHz one, we have other stuff to consider:

Either we got 256 KB or 320 KB, unclear without specific schematic for this unit, we may be able to upgrade the DPIP-32 packages to a 4Mbit chips, bringing the RAM in the daughter card to 1 MB total, which is hella expensive;

The 27C040Q-120 ROM is a 4Mbit (512Kx8) UV-EEPROM chip, we have one for the main software (P401A) and other for whatever it is (P402A), we may want to buy another EEPROM to keep that one as backup, and a larger one at it, since I think newer versions uses more than 512 KB of space;

It's nearly damn impossible to find firmwares for that thing, I was able to track some of them, but can't even say if they're adequate for my unit.

This complex seems to be upgradeable (if you ever find that stuff.) Here we can find:

• IC20: 27C040Q-120 (Firmware: P401A60905A), first firmware,
• IC21: 27C040Q-120 (Firmware: P402A), auxiliary UV-EPROM;
• IC24: CXK581000AP-10LL, or 131072-word × 8-bit High Speed CMOS Static RAM, which seems to provide our system with healthy 128 KB;
• IC25: idem, bringing the total amount of RAM in this module to 256 KB;
• IC26: OKI M6242B, an RTC clock chip;
• X2: 32.768 kHz crystal oscillator for the RTC chip;
• BAT: CR2354 battery, 3V, probaly of the /1GUF variant.

UPDATE (1/APR/2026): I used to assume the ROM were a 27C801, according to this post, however, it is not, as shown in the picture below.

• The RTC battery is probably long dead and showing signs of oxidation on it's legs; - There's was a bad and mysterious corrosion under the RTC oscillator, that I've already removed, after taking this photo;
• This same oscillator isn't operating reliably, sometimes it takes a while for the clock to start, well, clocking, and this component is probably the culprit;

UPDATE (1/APR/2026): I've been using the PBX for one year and the clock not only clocks, but also holds, having survived even a four-day period without power we had to endure thanks to Enel — FUCK ENEL! Heck, the last time I've updated the clock must've been over a year ago and it lost less than a minute since then. Truly remarkable.

Two proprietary parts are to be found here:

• IC2: PQVI620890F, according to TD816E service manual (that got the same part), it does «ASIC, Conferece, DTMF Generation, Tone Generation, P I/O Ports»;
• IC19: PQVI05JP031G, also appears on TD816E in the same position (IC19), albeit with different part number, as «Conf. & Tone ROM», also seems to be upgradeable.

Then we have:

• IC3: MT8981D, didn't look much into it, but seems to be a codec;
• IC22-IC2<unreadable>: TC51832ASPL-85, RAM chips, 32 KB each, don't know if they act as buffers for the audio system or if it's system RAM, TD816E also mentions RAM chips at IC22-IC23 and they seem to be connected to the system bus, don't know if the same holds true here;
• IC8, IC15, IC18: also look like CODECs.

If IC22-IC23(?) is really part of the system's memory, this brings the system to 320 KB of RAM, which we may agree is a very weird amount of RAM. Those systems are said to have either 256 KB or 1 MB, in the most recent revisions.

This makes the system kind of an hybrid between -1 and -2, hardware-wise. The CPU smells like 12 MHz, judging by it's model suffix (the datasheet for it mentions 8~16 MHz sub-models), but the amount of RAM is very far behind even other models of -1 (cf. «ROM mysteries» above.)

Anyhow, like I've posted on Sundance Communications, the oscillator seems to be doing 8 MHz, so I don't know if the CPU is really operating at 12 MHz.