28 September 2016

Olivetti Logos 262 PD desk calculator - keyboard must be there

While showing date and time is already an achievement, I wanted to make the display of my converted desk calculator look more dynamic so that it would drag the attention of bypassers. Showing hh:mm:ss is as easy as typing in the 6-digit number, press '+' '1' and keep on hitting the '+' key once a second.

While I could make it work interacting with the real keyboard, my first attempt at it with the emulated keypresses failed. The reason is that without a real keyboard connected some "control" settings are missing, especially the "print/don't print" line which defaults to "print every confirmation keypress" (like arithmetical operations, total, ...). This causes the printer go crazy, which delays the sum operation and ruins the effect (besides being annoyingly noisy).

So, next step will be reconnecting the keypad in parallel with my adapter board, adding a power switch to exclude my added circuitry and write a simple firmware to prove +1 can be done every second.

Since the "don't print" setting still activates the printer on fewer conditions (basically when a total is displayed), all uC-controlled interaction will probably have to rely on arithmetical operations: "clear screen" will be "multiply by 0".

Fast forward to Keyboard re-connected. I cannot switch off my added circuitry because the analog multiplex influences key lines and it behaves like a key is always pressed. So, everything must stay powered (at least the adapter board), and the switch must be handled by the Arduino firmware that has to know when it is allowed to take control of the calculator.

Next: I have to write a simple firmware to map the "T" key ("totalizer one"), so that the screen can be wiped without activating the printer (a double press on CE/CA clears "buffers" and prints something).

20 September 2016

Olivetti Logos 262 PD desk calculator - keyboard hookup

The keyboard of the Logos 262 PD is connected to the main board with a flat cable, all soldered. The way it was built is not convenient for attaching lots of wires in parallel and routing them elsewhere. So I opted for a more intrusive approach: unsolder the flat cable on the main PCB and install a row of 1" female headers.

Now I can plug the Arduino adapter board directly on top of the main PCB, or re-seat the original cable into it. Last but not least I left room on the adapter board to solder the original cable, making it in parallel to Arduino signals.
No more keyboard soldered. But wrong headers!

Notice that I used the wrong pin headers: these are optimal as IC sockets, but they do not accept male headers! I had to hack a connector, extract its thin pins and use them on the adapter board. I will keep this in mind for the next conversion!

14 September 2016

RTC drift and smartphone reference time

Once I uploaded the new firmware to my bedside clock, it was easy to figure out if the self-correction routine would work by comparing it with a time reference.

I elected my smartphone as the reference, but in a couple of days I noticed that the homemade clock was gaining time, much more than the expected 2"/day.

Well, it turned out that my smartphone clock was not properly adjusted. The setting "sync time with network" was unchecked. Not a big deal, if the smartphone wasnt't loosing time itself on a daily basis!

So, if you're trying to calibrate a cheap DS1302/DS1307 module, take long time measurements (one/two months) and make sure your reference is reliably in sync with a reliable source. :)

And, probably most important, don't do this serious stuff before going to bed, after a day full of the usual routine, family, work, stress, ...

12 September 2016

Olivetti Logos 262 PD desk calculator - Arduino hookup

Still working on displaying date/time on a desktop calculator (with VFD display). Lacking a keyboard bus, I have to emulate key presses via a microcontroller. How?

The answer came from the Net, of course. Since the original input closes a mechanical contact, I need to reproduce the same function with an electronic control rather than a finger pressure. The simplest solution would be to build a matrix of relays, but it is noisy, large and requires a lot of I/Os. A complex solution puts the [Arduino] in sync with the original microcontroller keyboard scan rate and modifies [Arduino] outputs accordingly. The clever solution instead uses two analog multiplexers connected back to back.

The CD4051/74HC4051 is an 8-to-1 analog multiplexer with output enable. If two of them are connected together through the common I/O, 8+8 lines can be switched (64 combinations) with just 3+3 control lines (2^3 = 8). Plus one Arduino I/O for the Enable control, equivalent to the keypress.

With this setup timing is not critical, and I only need to find out how fast my Arduino can "type in" new numbers, either experimentally or by measuring the keypad scan rate through with the visual help of an oscilloscope. I have already experienced that keeping a key pressed does not lead to multiple readings.

After a quick test on the breadboard I am building the adapter board. Arduino code will follow.

Breadboard test of 74HC4051.

05 September 2016

Taking care of RTC drift in my HP 5082-7300 clock

My HP 5082-7300 clock uses a DS1302 RTC module to keep the current time. That kind of cheap module is known to drift, and mine obviously does.

After three months of operation my RTC is about 170 seconds late. With due approximation this means is looses 2 seconds every 24 hours.

The temperature inside my clock box is quite constant, on the high side, thanks to heat dissipation in 4x HP 5082-7300 early LED tech displays. Temperature is one of the two main causes for drift, the other being the 32768 Hz crystal not properly matched to the chip specs.

I am planning a firmware upgrade that adds two seconds every day, in the middle of the night. New version will be uploaded to my github.