Friday, March 24, 2017

New VFD Filament Driver

I gave some serious thought to using the original transformer and duplicating the original power supply circuit to power my redesign of the P170-DH calculator's guts, but I decided it involved too many compromises. I don't need a +5V supply; the FPGA needs +3.3V and +1.2V. Since the filament and VFD drive supplies are derived from taps on the same transformer, adjusting the output voltage down from 5V to 3.3V would also reduce those outputs proportionately, and I have no idea what current it could supply. I gave some thought to using it solely to power the VFD while implementing separate +3.3V and +1.2V supplies, but that just seemed silly.

Monday, March 20, 2017

More VFD characteristics

I've pretty much settled on using the 12-digit vacuum fluorescent display (VFD) that came with the P170-DH calculator. Trying to squeeze a 2-line, 20-character VFD in its place is more work than I want to undertake. Besides, I'm having too much fun figuring out how to drive the original tube.

Tuesday, March 14, 2017

Canon P170-DH Teardown, part 2

Continuing with the teardown of the Canon P170-DH printing calculator, I'll look in more detail at the power supply.

As I mentioned before, the AC mains go through a 60Hz power transformer that brings the voltage down to 8 VAC (RMS). This then goes through a full-wave bridge rectifier made up from four discrete diodes and filtered with a large electrolytic capacitor. This results in an output voltage in the 10 to 11 VDC range, depending on load. This much was obvious; to learn more I really had to dig in.

Monday, March 13, 2017

Blocking Oscillator operation basics

I've reverse-engineered enough of the calculator's power supply to identify it as a blocking oscillator. Any electrical engineer would recognize this from intoductory classes, but I'm not a EE and had to poke around the 'net a bit to learn about its operation.

I often start with Wikipedia, but in this case the wiki article is terrible. It goes into great detail without first giving a basic overview of the circuit operation. Looking elsewhere I found a pretty decent description on the Integrated Publishing website starting here. The good stuff start several pages in, but I'll summarize below.

Saturday, March 11, 2017

Floppy failure

I continued my analysis of the power section of my Canon P170-DH calculator today, and recorded a series of oscilloscope screen-shots on floppy disk to document it. But when I tried to read the floppy, my USB floppy reader said the disk was corrupt. Bummer.

So I slid a new, unused floppy into the 'scope. It said the floppy was unformatted. What the...? Ok, format it! Format failure. I then tried to format another new one in my USB floppy reader, which refused. I even tried installing a real floppy drive in my Windoze machine, and that refused. Suddenly none of my floppy drives will read or write ANY of my floppy disks.

Has there been a global floppy disk failure? Sheesh!

What to do... what to do...

Wednesday, March 8, 2017

Canon P170-DH Teardown, part 1

I decided to start really tearing down my Canon P170-DH calculator. Here's the main board with the vacuum fluorescent display detached:

Sunday, March 5, 2017

P170-DH Vacuum Fluorescent Display characteristics

I've been pondering how to handle the display on the Canon P170-DH calculator I'm cannibalizing to provide keyboard input and printer output for my replica Busicom 141-PF. The Busicom calculator didn't have a display, of course; when you pressed an operation key the appropriate value was printed. The easiest thing then would be to dispense with the VFD entirely.

On the other hand, it might be nice to have the display to show what you're entering as you enter it. In a previous post here I gave thought to replacing the 12-digit VFD with a 2-line, 20-character VFD module from Noritake-Itron, which would let me display a lot of status information. However, though the height and width of the active display areas are compatible, the VFD module is much thicker than the tube of the built-in display. It's also heavier than I expected, so it'd need some structural support to keep it in place. Now I'm thinking it's more trouble than it's worth.

Another option would be to keep the existing 12-digit VFD and drive it from the same Spartan-6 FPGA that will emulate all the logic other than the CPU. This would require characterizing the existing vacuum fluorescent display tube before I remove it from the Canon calculator's electronics board. It carries a Samsung SSVD part number of INB-13SM36T but my attempts to come up with any sort of datasheet came up empty.