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Wednesday, 30 November 2016

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.

Logic Analyser

When I needed to look at waveforms on a digital interface recently, I used my Rigol scope as a logic analyser. It's not a mixed signal scope, but it has some features that allow it to be a 4 channel logic analyser. It can decode a parallel 4 bit bus as hex, for instance. It was right at the limit of what it can do, but as a logic analyser it was really at the bottom end of what I wanted it to do. So I looked about for logic analysers. There's a lot of the HP analysers about but they tend to need software on floppy disks and they will be decaying and get harder to find as time goes by, and they are mechanical and that is always the first thing to go.

Then I saw the Thurby Thandar (or TTi as they are now) LA4800 logic analyser.

It has it's firmware in ROM, and has 48 channels. It can go to 100MHz, but runs slower with normal pods. As an added bonus the service manual is on the internet, which makes it fixable.

I saw one on ebay and bought it, unfortunately without any pods. This isn't a massive problem as the service manual has the pod schematics, so I'm having a go at making a pod for it. Just a simple 32 channel pod, we'll see how it goes.

The analyser suffered a bit in transit and the display has a couple of odd faults on it, which is a real shame, maybe I'll find another one as a spares machine? Anyway it didn't start up when it arrived either, but it looks like it had a drop on the way here and the display connector had moved a bit, as after unplugging and replacing the display connector it works.

I'm currently routing a pod PCB...

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 as such a higher 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.

Wednesday, 23 November 2016


I needed a holder for my ER32 collets on the lathe, so one piece of scrap plywood and some holes and he have a holder:

The lathe has grown from just the machine to a machine surrounded by a myriad of stands and tools...

Thursday, 17 November 2016

Sharp EL-7050

I recently bought one of these calculators as it is a calculator of a type I didn't know existed until recently. It is a simple non-programmable with the added feature of a small ALPS four colour pen plotter. It can print out bargraphs, pie charts and tables of data that you enter into the calculator. I knew the ALPS mechanism was used by many manufacturers and many years ago I had a printer that used the mechanism.

It's a well known problem with these printers that a couple of small gears split with age and the mechanism dies. Unfortunately this had happened to the mechanism in my calculator as well. This stops the pen carriage from moving left and right and also the paper from feeding.

Some people have successfully fixed their mechanisms using small gears they have bought and others have 3D printed replacements. I thought of that but then I have a workshop for making things, so maybe I'll have a go at making some gears. I ordered some 4mm rod in various materials and went to set up the CNC 3020 ready to machine gears. Unfortunately the computer that drives it failed to boot properly and as a double whammy the spindle PSU has failed.

So, I decided to set up on the mill and do the gear cutting manually. I used the rotary table:

The cutter is a 0.2mm D bit that is one of the cutters I use on the 3020 for PCB milling. There's 13 teeth on the wheel, it has a 4mm OD and a 1.4mm hole for a push fit on the motor spindle. I worked out the 13 angles needed and cut a gear to a depth of 0.4mm or so. The dimensions are a bit rough as it's tricky measuring exact sizes with the manual calipers I have. I could use the spindle camera I suppose...

Anyway the gear was cut and I drilled a hole (1.2mm but enlarged manually, I had no 1.4mm drills, surprisingly as I have lots of drills of different sizes).
I managed to get the gear onto the carriage motor and it seemed to work, so I cut the second gear for the paper feed. Trying it out the papaer feed gear just didn't turn freely. I gave up and cut another one, this second one was much better:

I'll maybe cut it down to size later, it seems to work fine with the extra length, so I'll leave it for now. The carriage gear is here:

There's some oil from the mill on the wheel, it's actually white acetal.

The mechanism runs well enough to print out pretty well, here's the printer test (only two pens are working, I'll have to get some more somehow).

So the calculator now prints almost perfectly ( the carriage seems a bit too loose and rotates as it is drawing, not sure what th eproblem is there), and it's just as mesmerising as the plotter I had way back.

One problem that is left is that the LCD display has bad bleeding:

I have an ambitious possible solution for this, but it'll take a while to get to it I think.