DIY HP 30b WP 34s serial flash/programming cable

I recently bought an HP 30b with the intention of converting it to a WP 34s. The immediate requirement was for a cable to connect it to my PC in order to flash the Atmel AT91SAM7L128 with the WP 34s software. The manual indicates that the HP has a serial connector in the form of a set of six pads on the pcb located beneath six corresponding holes in the inner case of the calculator.

Wikipedia says this about RS232:

The standard specifies a maximum open-circuit voltage of 25 volts: signal levels of ±5 V, ±10 V, ±12 V, and ±15 V are all commonly seen depending on the voltages available to the line driver circuit.

Because the 30b serial interface runs at the same voltage as the calculator (3 volts) connecting it directly to the serial port of a computer would be a bad idea.

The MAX3232 (datasheet) is designed to be used in this situation. I had a breakout board (MAX3232 board) for this chip in my kit so I connected it up as shown here (connections). These little boards can be had from various suppliers on ebay for less than $4. My first attempts were disappointing - the PC software couldn't see the calculator. Reversing the Tx and Rx lines didn't help and a thourough check of everything else didn't uncover anything unusual.

The problem turned out to be the USB-to-Serial cable converter I was using. It works as expected when programming other Atmel chips (AVRs for example) but evidently this set-up is different. When I connected the calculator to a computer with a real serial port (and running XP) the problem went away and I was able to flash the HP 30b - which is now a WP 34s.

As you can see from the accompanying picture (dodgy pins) the method I used to attach the MAX3232 board to the calculator is very fragile. The pins are not located in holes in the pcb, and are likely to break off under the slightest strain. Others have devised much better solutions but I was in a hurry to get a result so I used the bits and pieces I had on hand. It's probably not necessary to mention, but just in case anyone thinks of drilling the pcb to make the set-up stronger -- don't; the pcb is (at least) double-sided and connecting the two layers would be disastrous. To position the pins for soldering I pushed them partway into a 2mm pitch header. Because they are closer together than the common 2.54mm I used wires terminated with pin connectors insulated from each other with heat-shrink tube to connect the MAX3232 board to the calculator.



Looking at the effort you had, I'd like to recommend Harald's USB converter board once more. Once installed, you'll have an earthquake-proof connection to your WP 34S whenever you need it.



If you plan to flash any more calculators in the future, you might want to look at this link. It shows how to make a very professional cable that is even better than the original serial port cables made by HP, because it works with a more modern USB connector. Serial connectors are getting much harder to find on computers these days.
Click Here

Edited: 4 Dec 2013, 11:16 a.m.

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