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Using a DMM in continuity mode might be giving you a misleading reading. Typically the DMM considers any resistance below about 200 ohms to be a contact. The 5V rail on the computer typically draws several amps, which translates to an effective resistance of no more than an ohm or two. In fact...

The NE558 quad timer chip makes me thing it's an analog joystick port. I'm not sure what the switch would be for though, maybe for selecting between analog and digital joysticks?

The hardware requirements for CP/M were a CPU that was binary compatible with the 8080, at least 20K of contiguous RAM starting at address 0000H, some form of console device, and some form of disk storage. It seems entirely plausible to assume that DRI would write all their software to meet that...

The first thing that jumped out at me is that / and \ are very similar, but do very different things. It's an easy mistake to swap them, and a pain to spot when reading the source. Most of the time the % is used for modulus. That conflicts with your number type designation though. You could use...

Lee Hart has a couple of kits available. https://www.sunrise-ev.com/#microcomputers I have both the 6502 badge and the Z80 membership card kits.

The E signal on the 6850 is a clock signal, not a static value. Invert the /IORQ signal from the Z80 and use that as your E clock. It's not quite what the 6850 expects, but it's close enough that it works on the RC2014 boards. Your transmit and receive clocks are way too fast. They need to be...

Underneath that plastic cover you will find a DS12885 RTC chip that's probably working perfectly. In order to make the DS12887, they soldered a crystal to pins 1 and 2 and glued it to the top of the chip. They then soldered a coin cell battery to pins 12 (gnd) and 20 (vbat). That coin cell was...

Any time I hear of 'lifetime' warranties, I'm reminded that the warranty expires when the company does.

One of these items from Adafruit can turn pretty much any 16x2 or 20x4 LCD module into a serial LCD. https://www.adafruit.com/product/781

RS-232 specified the allowable voltage range for all inputs and outputs. Your +12v and -12v are well within that range. The specification also indicated that the output drivers could be connected to any voltage from -25V to +25V without permanent damage. A device connected to your modified...

A picture of the top side might help. The identity of the component the capacitor is soldered to would be a good clue. All the Apple II motherboard pictures I can find show a bunch of ferrite filters there, so more than likely that capacitor is for extra noise filtering on one of the power...

CP/M abstracted the hardware away almost completely. The requirements were a CPU that was binary compatible with the 8080, at least 20K of contiguous RAM starting at address 0000H and a disk driver that worked in 128 byte blocks. Everything else was hidden from user programs. As long as the disk...

Missing the U jumper would explain the problem. The fact that it ran the monitor was enough to verify that the data bus was correctly working. It also indicates that at least a large part of the addressing is working. My immediate suspicion was one of the high address bits stuck, which would...

It works just fine when I try it on my badge. I get a memory size question and hitting the enter key gets me to a ready prompt. Possible things to check: Did the ROM contents somehow get corrupted? Are all the address pins on the ROM properly in the socket? Is there a bad solder connection on...

Normal practice would be to have one of the inner layers as a ground plane, and the other as a power plane. The very thin separation and the large area of the planes effectively turns the entire board into a giant decoupling capacitor.

The Z80 IN and OUT instructions duplicate the same instruction in the 8080. This means that not only do they only accept an 8 bit value for the port address, in operation they also put the contents of the accumulator onto the upper 8 address lines. In a system with 16 bit addresses for the...

Back in the 1980s the college I work at had three Vaxen. An 11/780 running a classroom full of CAD workstations for the architectural students, an 11/750 doing tool path calculations for the CNC machining students, and a second 11/750 for the electronics students to use learning microprocessor...

I have an Epson HX-20. 8 bit 6301 CPU clocked at 613KHz

You might recognise a couple of these...

From on the low end of computing power, this is one of my vintage trainer boards. I have a few trainer boards for different processors.