HP48RamCard Repository

A 128K RAM Card for the HP48SX/HP48GX Calculator

This repository contains the schematics and Gerber files needed to create a 128K RAM card for the HP-48SX/GX calculators. Data sheets for the parts used are also included. The Bill of Material calls out specific vendor and vendor part numbers for the passive components used, but any equivalent part in the SMD 0805 package can be substituted.

The PCB board dimension is 2.125" wide by 3.375" high. The board thickness is 0.8 mm (1/32") and a 0.8 mm plastic strip must be added so that the card makes a snug fit in the calculator connector. A PCB thickness of 1.6 mm can be used instead if a direct fit is desired, but many PCB houses treat this as a special order size and charge accordingly. If you take this route be sure to notch both sides of the PCB at the connector end for a distance of about 1" and depth 0.1" in order to clear the latch engagement pin.

Battery backup is by use of a 12 mm lithium coin cell, either the CR1216 or the CR1225. The CR1225 has the higher capacity rating of the two at 48 mAh. Although that's only half the capacity of a CR2016, the Cypress MoBL SRAM memory device only requires 1 uA in standby mode when the memory is not selected so battery life should be reasonably long.

Repository Content

The Board directory contains the schematic and PCB board images. Gerber files are in the Gerber subdirectory. The Datasheets directory contains relevant device information, copyright the manufacturer of course.

Assembly Notes

The flexible contacts punched from the top of the battery holder are bent downwards far enough to touch the PCB. Left as-is they will make it more difficult to insert and remove the battery. The spring force can even be enough to cause the solder bond between holder tab and PCB to fail resulting in the battery popping out of the holder. Press the two spring tabs upwards so that they are closer to the top of the holder.

The battery holder is held by solder to the PCB. Be sure to apply enough to cover the contacts well because the holder will likely be subject to a pulling force by those who grasp it to remove the card from the calculator.

The memory protect switch is a more suitable point to grasp the card when removing it from the calculator. The switch is anchored at its four corners by solder tabs. In addition, two pegs extend from the bottom of the switch body into holes in the PCB. Pull upwards in the plane of the PCB.

The top of the battery holder is at the potential of the positive side of the battery. The holder top presents a possibility of a short, so it is advised to cover it with a strong adhesive tape or contact sheet. Better yet, remove the chrome coating from the top with rough sandpaper prior to soldering the holder to the board, then coat with paint or nail polish after the soldering operation.

To insure the card contacts are pressed firmly against the card connector contacts in a calculator, attach a strip of 0.8mm (1/32") thick plastic behind the contacts on the component side of the PCB. The strip should meet the lower edge of the PCB and extend upwards about 1/2", but should not extend the full width of the PCB. Keep the strip about 1/4" from the left and right sides so that it doesn't cause the card to be blocked by guide tabs in the card connector block inside the calculator.

Usage

The card may require a bit of wiggling to clear guide tabs molded into the card connector assembly. These tabs help guide a commercial card and open a spring-loaded cover protecting the card contacts. Insertion and removal is fairly easy in either card slot of an HP48SX. It may help to turn the calculator on its face and then some downward pressure applied to the back of the card as the card is pressed down on the top edge.

Insertion in slot 1 of an HP48GX is slightly "stickier" but is easily done by turning the calculator over. Slot 2 is a different story as the bottom edge of the PCB catches on a ledge above the connector slot. A plastic ruler can be use to press the back of the card near the bottom until it clears the ledge. An alternative is to move the plastic strip from the front to the back of the PCB just above the row of contacts. The downside is that the card will now no longer fit in slot 1.

Project Status

A set of boards were built and tested in HP48SX calculators and outside of a calculator shell. All cards showed an initial decline in battery cell voltage before stabilizing around 3 Volts. All cards maintained their content across the multi-week test period.

The pullup resistor R1 on the Output Enable input to memory was found to be unnecessary, but did not affect battery life during the test period. I recommend not installing R1 on any board build.

Improvements to the design include using a smaller contact pad for the battery bottom since the current size presents a slight risk of a short between the bottom pad and battery housing if a metal probe is inserted in the general area. Some designs show a 2.2K current limiting resistor in series with the CdDet input. A resistor could be added to the design and a zero Ohm short could be used if the resistor proves to be unnecessary.

The supply voltage to the switch when in the RAM position could be VccOn from the calculator rather than the memory device supply Vcc. No detrimental effect has been seen with the arrangement, but if battery life is compromised as a result, the switch should be moved to the ROM position when the card is not in use.

Bill of Material