u/Bits_Passats

Greetings!

Sorry, it will be a wall of text... but I promise you I will try to keep things interesting. Also, if someone has something to say, either because something is weird or because he/she has additional information, I would be willing to hear in the comments, please. That being said, let's go!

========== Context ==========

It all started in 2022. I was going to kill my father, who bought a non-working microcomputer. I was interested on it as well, but I that thing was something that, at the time, I didn't know how to manage. I am talking about an IBM System/23 Datamaster, model 5322-124. Don't worry for my father, I spared his life even after buying ANOTHER AS/400 9404 without telling me and he is doing fine right now.

That being said, an evaluation of the board started, and the removal of the original ROMs was performed in order to test and have backups for in case of failure. At that time, there was only another set of this computer's ROMs available, but was incomplete due of one of them being faulty. And a miracle happened, all of ours were fine.

Eventually, we repaired the computer. It was an insignificant failure that prevented the computer from performing any action at all. However, with all solved, it started flawlessly. After 45-50 seconds of turning the power on, finally our Datamaster BASIC prompt was printed on screen.

Then it was another era in which my father almost got killed, both by depression episodes and heart attacks. The cause? I removed the motherboard and started performing tests on it, in order to satisfy my curiosity. And I was always taking notes. We are very similar in character, but we have different approaches to handle the same problems. He wanted to leave it as-is, while I wanted to obtain data and even considering to take risks. He had some scares and even cried every time I messed with the motherboard... Ah... funny times.

However, it was in 2025 that action really started to escalate and pay off. I started a MAME driver in February and had it working after a couple of months. That gave me insight of the internals and even sparked more repairs. But the best part was to come.

========== Prototype ==========

My curiosity was still hungry, so I decided to look at a more wide perspective. What does this means? Well, there was the generally-accepted notion that the Datamaster was the origin of the 5150 PC. But at what degree? How much is from the old design and what was from the next one? Since the Datamaster hardware hadn't had a serious overview before outside IBM, nobody could tell. But there other ways.

So I made a plan! A very cunning one indeed... It involved in makind a three-way comparison between the Datamaster, the 5150 PC and the PC prototype, so in that way I would be able to see even elements presents in the original idea of the PC that would be later dropped for production. And to top that, I successfully established contact with one of the twelve engineers linked with the PC development. He told me very interesting things about the Datamaster and the transition to the PC. Additionally, I have recently obtained a copy of development notes about the PC, that clarify a few parts of the development and explain various choices.

And I saw something nice at the prototype that explains various things from the PC... the memories! The System/23 employs a derivative of the TMS4116 called TMS4132. They are basically TMS4116 (same die) with two extra pins for RAS/CAS. Two slightly different variants were made so the new pins were NC on one and used at the other, then TI soldered in piggyback in the factory - and they said it was a 32K memory. However, it was quickly replaced by the TMS4332, which has both dies in the same package and maintains pin-compatibility with the TMS4132. So, the Datamaster word-processing card employs TMS4332, as well as the Apple ///. And... they can be clearly seen in pictures of the prototype board! The original PC employed the same kind of memory, the TMS4116, as DRAM. But they refrained from using the 32K ones and stuck with the original 16K ICs. In the original notes I found out that they originally wanted to make two PC lines and one of them might have had no RAM on board, but DRAM cards just like in the Datamaster. Other parts of the notes suggest they wanted to mix TMS4332 and TMS4116, but also found that they were expecting 5V-only memories to appear, so in my humble opinion I think they ditched the 32K ones to wait for the TMS4532 to appear. However, if 5V memories had started to appear, why didn't they adopt the TMS4164 from the start?

There is an explanation for the last question. The Datamaster employed the Intel 8202 to automate the memory refresh. The 8202 was limited to 7-bit wide multiplexed addresses (14-bit, 16K long). The PC team wanted the Intel integrated chip to be in the PC chipset, and this can be seen in the development notes. However, if this is indeed true, why the PC is so different? The answer was given to me by my contact. During late development a defective batch of the DRAM control circuit was delivered to IBM, so with time ticking, they had to resort to the current mechanism of PIT + DMA. And this way, they adopted the makeshift solution for the final design, effectively removing the controller and also lowering the costs.

The chipset of the Datamaster changed a little when they built the prototype we currently know. It maintained the 8253 PIT and 8259 PIC, also had the 8255 PPI, although with other functions than in the first computer. The 8257 DMA was replaced by the 8237. There has been a lot of talks of why the x86 and not other processor families. However, my contact clearly said that they wanted to employ the Intel chipset of peripheral ICs, as well as a 16-bit microprocessor, and the only option they had was the x86 family of processors. They also wanted almost full compatibility with the Datamaster expansion bus. This means that if you want to blame something for the x86 architecture adoption, blame the Datamaster, which is what put the selection conditions.

There are other elements I could explain about the trilateral comparison, but this is becoming very long and I want to pass on the next subject. So if you allowed me, I would like to pass on the next subject.

========== The unwanted child ==========

The Datamaster has another son other than the PC... Although we could also count it a PC descendant too. Both the 5324 and this one have elements that were discarded from the PC development PC, as well as some elements from the Datamaster and also the PC.

I am talking about the IBM 5247. From the System/23 line of computers, it is listed as a peripheral, an intelligent HDD that can be connected with up to four Datamasters in a star topology. We could be bold and call it the grand-father of NAS devices.

Recently, I was able to take a look at its motherboard thanks to an acquaintance, and I was left very surprised. I got most of the raw specs just from the pictures. So, that embedded computer is more powerful than a Datamaster - no surprises here - but also more than a PC XT. It employs also the same CPU as the Displaywriter, a full-fledged i8086. This means that memory accesses from ROM and from and to RAM are all 16-bit wide. It has 32KB of ROM structured in 16k x 16, and was sold with 64KB/128KB of RAM, although the pinout of the memory cards hint they can make increments of 32KB up to a max of 128KB. As said, they are structured on cards like in the Datamaster. Other interesting things are the presence of a 8253, 8259 like in the PC and S/23, 8237 like in the PC. It also lacks the 8202 like in the PC, so a PIT + DMA refresh mechanism is suspected. The RAM chips employed in the RAM cards are also TMS4132. The expansion bus derivates from the Datamaster, and therefore it is very similar to the PC one.

The hard disk controller card is built around a 8237 and is just as big as the whole motherboard. It is connected to the computer motherboard through another, custom-designed bus, which could be confused with the expansion bus.

In any case, the thing seems to be a byproduct of the IBM PC prototype built with the very same doctrines of the Datamaster.

========== Last words ==========

Well, sorry again for the wall of text. I hope you have enjoyed it. If you need to know anything, just ask in the comments.

I would also like to show my appreciation to all the people who have, directly or indirectly, helped during that investigation. This hasn't finalized, there's always things to learn!

Last but not least, I let you know no father has been harmed during the course of this report.

========== Sources ==========

About the Datamaster:

https://bitspassats.com/index.php/IBM_System/23_Datamaster

About the PC prototype:

https://x.com/TubeTimeUS/status/1333231389865517057

About the PC development notes:

https://forum.vcfed.org/index.php?threads/ibm-5150-prototype-drawings-plans-from-1980.1256591

About the 5247:

https://forum.vcfed.org/index.php?threads/not-a-pc-not-a-system-23.1257298/