Studying the PCB of a 70s calculator. What were they thinking? Can we ever know? - Page 1
Forum section: electronics / beginners

author: futurebird
author_rank: Newbie
timestamp: April 29, 2016, 10:25:19 am
content: I've been studying the PCB from an ELSI 8002 calculator from 1974. I like older electronics since I can understand them more quickly. Also, I'm thinking of repurposing the case and keypad for a project, though now that I've fixed it (by re soldering the battery connectors) I don't know if I can bear to pull it apart again. It's so lovely. (sniff)Maybe, I'll buy a more deeply broken one for my project... (I'm super excited about this project, but I don't want to share too much about it until, I've got more of it in place. O0 )Sentimentality aside, I'm rather confused by the layout of the keypad. The keypad initially looked like a typical matrix keypad, but after carefully studying the traces I've found that it isn't using rows or columns.At first I thought this might be because they were trying to save pins on the micro controller. A matrix layout with n row and m columns requires n+m pins. But, really, we only need a unique pair of pins for each button. So, really we only need x pins where n*m <= x Choose 2.A 4x5 matrix has 20 buttons and 20 <= 7 Choose 2 = 21. (really only 18 buttons are needed since the reset button "C" is mapped in a special way and shares no pins with the other buttons, and there is an unused pad, though maybe it's used in other models?)I thought this was what was going on since rows and columns don't have a common pin... but the layout uses 9 pins... ? With 9 pins why not just make it a matrix?I don't know what's going on, but it's still a lovely device...

author: Whales
author_rank: Super Contributor
timestamp: April 29, 2016, 11:39:31 am
content: Perhaps an expanded model was planned?

author: MK14
author_rank: Super Contributor
timestamp: April 29, 2016, 11:57:23 am
content: Some/many calculators, may have shared the seven segment display output pins, to simultaneously scan the keyboard as well. (To save limited/expensive IC pins).So if there are 9 separate digits (I'm NOT 100% sure from your photographs), that might be why. (Many calculators have 8 digits. But a quick glance at yours, seem to show a ninth (on the far left), just for + or - signs, or something ?).Seven segment gives 8 output lines as well (7 + 1 for the decimal point), or even 9 if it has decimal point AND a comma (on some calculators).

author: MK14
author_rank: Super Contributor
timestamp: April 29, 2016, 12:47:10 pm
content: I could not find the EXACT chip you have in your calculator. But the following one, I did find, also has 9 pins (or 10 optionally), for the keyboard matrix, in one dimension, just like yours. It also apparently shares those pins with the display (like I mentioned, above).http://html.alldatasheet.com/html-pdf/118646/TOSHIBA/TC83220-0027/1214/4/TC83220-0027.html

author: futurebird
author_rank: Newbie
timestamp: April 30, 2016, 06:19:20 am
content: I asked about this at stack exchange and I have an intriguing possible answer.Asdfexat the engineering stack exchange suggested re-ordering my table as follows:*HFGBDA13579C24680E.UNB%CEIXU÷-=U=Unused PairNB=No buttonOn the surface this ordering brings some logic to the surface, the last two rows are not nearly as clearly ordered so there could just be a coincidence here.Original map:MK14, thank you for the information about the possibility of the display sharing pins I was able to trace a few of the pins from the 28V VFD to the micro. (but those connections are not the 28V ones, they are at 2.5V, the chips seems to have 5V logic. I would like to use the vfd in my project but I'm still looking for a datasheet or any info to see if it makes sense. I basically want to use everything except for the micro which isn't programmable and seems to have no documentation.... Maybe I should just remove it from the PBC and wire the holes it leaves behind to my micro dead bug style? I wonder how big of a nightmare that would be?Do you think doing a circuit diagram of the board as it is now is a good first step?Has anyone ever done a similar project

author: MK14
author_rank: Super Contributor
timestamp: April 30, 2016, 10:33:31 am
content: Making a circuit diagram makes a lot of sense. Even if it is not 100% complete.The keyboard matrix, may also be the way it is, to allow, multiple keystrokes (at the same time), to be successfully handled. Some matrix layouts, CAN'T readily handle some keystroke combinations (n-key rollover), which can happen, if very fast calculations are pressed on the keyboard.Because it is *just* a simple calculator, I would have thought that it should still be ok, WITHOUT complicated n-keyrollover.It could also be that way to save money. There might be diodes and/or resistors, along one or both axis of the matrix. So some X by Y's, are cheaper, component wise. Pennies can matter in calculator component costs.The microcontroller may have limited/restricted I/O, making the matrix layout the way it is.E.g. there were too few outputs.Modern microcontrollers, have almost unlimited outputs, but very old microcontrollers may be a lot more limited.E.g. Some pins could be input only, and connect to internal interrupts/timers/capture units etc.If I recall correctly, there were some "standard" calculator chips, flying around at the time. So any particular one, would be relatively fixed, in its input/output configuration, because the hardware IC design was FIXED, at that time.