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A 512KB RAM Card for the HP48GX Calculator

This repository contains the KiCAD design files for an HP48GX calculator RAM card. The card contains a single 512KB SRAM chip and can be used in either slot 1 or slot 2 of the calculator.

When the card is used in slot 2 it makes available 4 x 128KB banks of storage that appear in the calculator as ports 2, 3, 4 and 5.

When used in slot 1, only one of the 128KB banks is available, which will appear in the calculator as port 1. Two switches, mounted on the end of the board and accessible when the calculator card cover is removed, allow control over which bank is used.

Prototypes

The card is currently at revision 0 (initial design). It has been prototyped and found to be fully functional, but has a few issues that could be addressed, if anyone wishes to do so, before further boards are fabricated (see below).

A PDF of the schematic is available in doc/schematic.pdf.

Some photographs of my prototypes are also available:-

I manufactured the prototypes at pcbway.com, selecting a yellow solder-resist, black silkscreen, immersion gold finish and gold fingers with a bevelled edge. The gold finish is necessary to avoid corrosion on the battery pad and the connector fingers.

However, I also made a mistake: I ordered a 1.2 mm board thickness because I had read somewhere that this was used by the original HP48 RAM cards. Unfortunately 1.2 mm IS TOO THIN -- the cards do not seat properly in the connector. I had to pad the back of the cards with several layers of sellotape (transparent sticky tape) to get a firm fitting. I believe that a standard 1.6 mm board would be the correct thickness to use (with a bevelled edge for easy insertion).

Hand soldering the components onto the board is not too difficult (with a fine-tipped soldering iron and a magnifier) -- I selected relatively large footprint devices specifically to make assembly easier. The parts list is included below.

The current consumed by the CR2016 backup battery when the board is outside of the calculator is less than 3 microamps, giving an expected shelf-life for RAM contents of over 3 years for a 90mAh capacity battery. I have yet to measure the current when the card is inside the calculator and the calculator off (need to solder some wires to do this), but hopefully it will not be much different.

Operation

For use in slot 2, set the SW2 and SW3 switches (marked A18 and A17 on the silkscreen) to their central positions (marked A for Automatic), and set the SW1 switch (marked PROT on the silkscreen) to W (for Writeable). After inserting the card into the calculator, issue the PINIT command and you should see ports 2 through 5 become available in the ports list.

For use in slot 1, set the A17/18 switches to fixed 0 or 1 positions. You may keep four different port 1 configurations in this way. Just be sure to switch off the calculator before moving the switches.

I have tested the functionality of the boards with the CHKMEM utility available from http://www.hpcalc.org/details/7394, and also with a few weeks of general usage. Initially, I had some strange issues with one of the boards that I eventually tracked down to some dodgy soldering. Other than that, I have not found any major problems so far.

Note that I have not (yet) tested the card in an HP48SX calculator, but I am hopeful that some combination of switch positions will allow it to work in this calculator too.

When it comes time to change the backup battery you will notice that the board must be removed from the calculator, and so there will be no battery power available to power the SRAM chip during the battery change process. This is the reason for capacitors C1 & C4, they should be able to supply enough current to support the SRAM during this procedure. Do not take too long though. My own testing assumes 10 seconds is long enough to change a battery -- have the replacement battery to hand and, of course, make a backup first!

Revision 0 Issues

I have identified the following issues with revision 0 of the design:-

  1. The switches could be a little closer to the edge of the board, allowing them to be more easily operated when in-situ in the calculator.
  2. The C2 and C3 silkscreen legends are swapped.
  3. The C4 silkscreen legend is lacking a '+' sign.
  4. Removal of a card from a calculator can be awkward. Consider the addition of a small hole to allow hooking the card out with a paper clip.

Parts List

Part Description Digikey Part Number
R1 RES SMD 1M OHM 5% 1/8W 0805 311-1.0MARCT-ND
R2, R3, R7, R8, R9, R12 RES SMD 47K OHM 5% 1/8W 0805 311-47KARCT-ND
R5, R6 RES SMD 10K OHM 5% 1/8W 0805 311-10KARCT-ND
R4 RES SMD 4K7 OHM 5% 1/8W 0805 311-4.7KARCT-ND
R15 - R22 RES SMD 330 OHM 5% 1/8W 0805 311-330ARCT-ND
C1, C4 CAP TANT 33UF 6.3V 10% 1206 478-8527-1-ND
C2, C3 CAP CER 0.22UF 10V X7R 0805 445-8190-1-ND
D1 DIODE ARRAY SCHOTTKY SOT23-3 BAT54CFSCT-ND
Q1 TRANS NPN 45V 0.1A SOT23 BC847C-TPCT-ND
U1 IC SRAM 4MBIT 45NS 32TSOP CY62148GN-45ZSXI-ND
SW1 SWITCH SLIDE SPDT 300MA 6V 401-2016-1-ND
SW2, SW3 SWITCH SLIDE SP3T 300MA 6V 401-2017-1-ND
BT1 CR2016 3V 90mAh COIN CELL N/A
Battery Holder RETAINER 20MM COIN CELL SMD 36-3026-ND

Licence

The materials in this repository are released under a Creative Commons Attribution-ShareAlike 4.0 International License. This means you can do what you like with this material as long as you acknowledge my contribution and distribute any changes you make under the same licence. See cc-by-sa for details.

Paul Onions paul.onions@acm.org

22 June 2017