Epson HX20 Interface

One of the nice things about older hardware is that a lot of it comes with expansion connectors or other ports that allow you to attach arbitrary hardware to it. The HX-20 is no exception and comes with an expansion port which allows access to the bus signals of the main processor (yes it has two - almost dual core). I wanted a simple IO port so breadboarded a simple 8 bit output latch and 8 bit input circuit using old fashioned 74 series logic.

As a more permanent solution I routed a PCB.

I then used an old PC HDD cable to attach to the HX20 which helpfully uses a standard 0.1" pitch IDC connector. Unhelpfully they have used a different numbering scheme to the rest of the world which means the pin numbers are all jumbled, but that's not and unsolvable problem, just an irritation.

The output port can be used to drive other hardware, I'm going to use this one to drive a relay module:

The bottom two bits of the data bus are wired up on this PCB, so you write to address &H5000 and D0 and D1 will end up latched in the output port. To read inputs you read the same memory location, again D0 and D1 are the data bits that are wired up.

Sharp PC1211

Sharp PC1211

This one was iconic back in it's time. It's still a nice small BASIC pocket computer, but the initial problem I had with it was the batteries. It uses 1.35V mercury cells originally, and they aren't available any more as mercury is very naughty.

Most modern button cells are alkaline and generate 1.5V, so putting four of those in this machine would result in 6V, which is quite a bit higher than the 5.4V that the mercury cells would have generated. It's probably going to cause a failure at such a voltage. Fortunately, if you add a diode in series with the batteries you can get 5.4V from 6V as 0.6V is the standard diode forward voltage drop. I also have a service manual for this machine and looking at the circuit diagram it looks like there's no centre tap taken from the batteries, so the lower voltage should work.
I soldered a diode in to the machine, which was easy as there's plenty of room available:

The new diode is the one to the right of the diode with the yellow stripe on it. It's a bog standard 1N4148 I had in stock. After fitting some LR44 batteries, the PC1211 started up:

Jolly good news. I'll see how happy it in in the long term.
The printer is a different story and for now hasn't sprung into life. There's an internal battery pack which looks remarkably well considering it's age, so I'll try recharging that for a while. If that fails, then I'll swap the batteries for new ones.