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[WP34S] HP cable vs USB adapters compatibility question - Printable Version

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[WP34S] HP cable vs USB adapters compatibility question - E.Lub_EU - 09-26-2011

Hello all,

I am looking for suitable USB-serial cable/adapters for the 20b/30b.

Looking at the photo of the DE9 pinout on page 2 in the document "Interfacing to the HP20b V3", I am surprised to see the Gnd (labeled Gnd2) on pin 9, where I expect a signal: RI (Ring Indicator).

The FTDI cables & adapters feature a Gnd connection on pin 5.
I have checked the following documentations at FTDI:



I would like to know

- Can people using one of these confirm that they work,

- Can somebody with a HP serial cable confirm the cabling on the RS232 side ?

Thanks in advance


(Edit) changed title

Edited: 26 Sept 2011, 6:34 p.m.

Re: [WP34S] HP cable vs USB adapters compatibility question - Marcus von Cube, Germany - 09-26-2011

There are several possible solutions:

1.) Straightforward: Get an FTDI equipped USB / Serial dongle with a 9 pin male D-type connector and connect the HP programming cable to it. This should work out of the box.

2.) Make your own, bypassing the current drawing level-shifters:

a) Get an FTDI TTL-232R-3V3 cable which ends in an 8pin connector or just 8 wires. See this page: http://www.ftdichip.com/Products/Cables/USBTTLSerial.htm

b) Open the HP flashing cable, remove the RS232 cable from it (the side with the 9 pin D-connector) and attach the corresponding wires of your FTDI cable to the far end (directly to the cable going to the calculator.) You should disconnect the power from the level shifter IC or desolder it completely. The ERASE switch needs the 3V connection because the erase input is active high so you need to cut the trace at the right position. I plan to do this myself and will document it when it's working.

Re: [WP34S] HP cable vs USB adapters compatibility question - E.Lub_EU - 09-27-2011

Hello Marcus,

Thanks for your elaborate response.

I have come exactly to the same conclusion, and am preparing an order from FTDI on the same line:
- one "plain" USB-RS232 to use with an unmodified HP cable,
- one USB-"3V-TTL-serial" with an open end cable.

I had come to the same implementation idea (cutting the wide trace between the switch and the level shifter), cabling direct from the FTDI cable to the calculator side of the HP cable. Big advantage is that there is no need for another PCB inside the HP "button box".

Some possible caveats could be:

- the 3V3 signals from the FTDI chip will exceed hte Vih spec, being above the Vbat level.

Probably just ok with fresh batteries, unless the 20mA drain causes them to sag significantly. Has this been measured already?

With partly depleted batteries, one would actually offset the SAM7 internal Vdd plane value through the Vddio (bit of speculation here, as the internals of the SAM7 are not that detailed), I believe that keeping this condition for a long time might even damage the chip.

There might also be a problem with the reverse (charging) current spec of the batteries, like when leaving the calc in low-power state with the cable connected (USB powered)

This does leave the calculator running from ts own batteries, and it was mentionned here that they drain fast under these conditions.

So I was toying with the idea of actually powering the calc through the cable (same work on the cable PCB, plus connect Vbat to the 3V3 from FTDI).

This implies some constraints on the user:

- removing both batteries before plugging in the cable, so saving RAM to flash is mandatory befor starting a serial session,

- same before exiting the serial session, as the cable has to be disconnected before any battery can go in,

- a slight risk of losing power to the usb if/when the PC goes into standby. Aprat from changing the PC settings, this can be avoided easily by powering the USB cable through a powered hub.

This solution might appeal more to a developper (long sessions with a PC connection), that an "normal" WP34S user.

Sorry for the lenth of this repliy!

I am still puzzled by the Gnd on pin 9 in the mentionned document though. Will check when I get mine.

Thanks for your help,

Re: [WP34S][Solved] HP cable vs USB adapters compatibility question - E.Lub_EU - 09-27-2011

Hi all,

I just tested the cables I received from Gene today. They are naturally compatible with the standard 9pin serial pinout.

Anybody interested in these details, please amend your copy of the pdf document "Interfacing to the HP20b V3". On page 2, look for the photo of the connector (bottom right) and change the Gnd indication so that it points to pin 5 (top row, leftmost).

Hope this helps!


Re: [WP34S] HP cable vs USB adapters compatibility question - Marcus von Cube, Germany - 09-27-2011

According to my measurements, the FTDI output is always below 3V3, more around 3V. You can add a small resistor on the transmit line.
Powering the calc through the cable will definitely be an option for my development device.