This system got not more than a few hundred KB's of RAM. System configuration is stored on a EEPROM. The CPU is a 68k 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 unclear, 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

However, this document contradicts those informations, since this ROM does have a lot of features that would make it a -4 model. Oh, and the CPU seems to be a 12 MHz model, as well.

In reality, it shouldn't matter much, unless you need specific features. In my case, KX-TD193DX Caller ID card won't work with P401A ROM. [Put the 193DX investigation in a separate article]

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

• IC20: P401A (P401A60905A), first firmware, UV-EPROM, probably 27C801, according to this post;
• IC21: P402A, auxiliary UV-EPROM, probably also a 27C801;
• 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.

• 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;

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.)