A one-bit processor explained: reverse-engineering the vintage MC14500B

It’s very suspicious to me that it was “not specified” but the opcode list has holes exactly where you would expect the ROR instructions to go. I want to see the ALU and decode ROM PLA to figure out the truth about what was taped out. Did a team of dozens really forget to add ROR but included SHR, ASL, and ROL? Or did they make a mistake in the mask?

It’s implied in the article I think.

> In 1977, the MC14500B cost $7.58 in quantities of 100 ($32 in current dollars), which seems expensive. However, at the time, an 8080A CPU cost $20 and a Z80 cost $50 ($85 and $215 in current dollars) so there was a significant cost saving to the MC14500B.5 However, the steady fall of processor prices soon made the MC14500B less attractive.

Perhaps in 1977 you couldn’t source the 4004 in the quantities you might need to make a product leaving you only with more expensive contemporary processors?

See my post from above. In industrial control there are a lot of processes with a large number of on/off sensors and actuators.

One-bit programmable controllers like 14500B were good enough to handle those.

In such old installations, the sensors and actuators that could not be handled by one-bit controllers were handled by analog controllers, unlike in modern installations, where a MCU handles digitally not only the Boolean variables but also the analog values through ADCs and DACs, so arithmetic computations are also required, besides the logic operations that could already be performed by something like 14500B.

I was early in my career at the time, and remember looking at the datasheet and asking myself the same question. Reflecting back, I think the sibling post that mentions an upgrade of an older discrete TTL PLC system could be the bulk of the target market. So while it was unlikely someone would begin a clean new design with a 1-bit CPU, there were older systems that already had the 1-bit architecture that suited this device for a mid-life re-do.

Besides that, semi vendors have on occasion been known to manufacture a device that has no market success to due to poor product/market fit, or technology bypassing it by the time it was finished. Most devices were originally made as custom designs for some specific customer or application so perhaps there's a boatload of these in some 1970's car.

Possibly, but I suspect this chip would probably be more of a tangent than something on the path to a full processor. Also, a 1-bit processor isn't very exciting as far as applications, compared to even a 4-bit processor.

Traditionally you'd start with 74181 or derivatives, or if you wanted to go all out: Am2901.

The transmission gate is pretty common in microprocessors. In NMOS circuits, it's a single transistor called a "pass transistor", but in CMOS it uses two transistors and is usually called a transmission gate.

It takes a logic signal as input, as well as a control signal. If the control signal is 1, the logic signal goes through to the output. If the control signal is 0, the transmission gate is disconnected. You can think of it like a relay-controlled switch, or a tri-state buffer.

The nice thing about MOS circuits is the gate resistance is almost infinite, so if you open a pass transistor, a gate on the output side will keep the old value (for a few milliseconds at least). So you can create latches almost for free. This is used very often in microprocessors. The disadvantage is the chip has a minimum clock speed, or else the data will leak away.

Pass transistors / transmission gates can also be used to implement multiplexers, selecting one of the inputs.

A disadvantage compared to regular logic gates is that a logic gate amplifies the input signal, while a pass transistor weakens the input signal. So you usually can't connect two pass transistor circuits together directly.

https://en.wikipedia.org/wiki/Pass_transistor_logic

The complete schematics of the Xerox Alto are on Bitsavers. I don't think photos would help a lot with generating a PCB, since the chips cover much of the wiring.

A bigger problem with a Xerox Alto replica is the wire-wrapped backplane. It would be a pain to redo that by hand.

For most purposes, you'd be better off using the ContrAlto emulator.

Schematic link: http://bitsavers.org/pdf/xerox/alto/schematics ContrAlto link: https://github.com/livingcomputermuseum/ContrAlto

I originally mentioned ladder logic briefly, but I figured nobody would be familiar with it, so I cut it out. The documentation discusses ladder logic a lot (as you remembered), and how to convert from ladder logic to the MC14500B: http://www.bitsavers.org/components/motorola/14500/MC14500B_...

Even today, Harvard architectures[a] like the PIC have different instruction and data widths. For example, the PIC12 has 12-bit code words with 8-bit data words IIRC.

[a]: Are there von Neumann architectures with different width code and data?

It's still true today.

The ubiquitous 64-bit ARM has 32-bit instructions and 64-bit data registers.

even more extreme are ZISC: https://en.m.wikipedia.org/wiki/No_instruction_set_computing