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Does anyone have a link to the technical documentation on the HP-30B
that was used to make the serial communication mods?

I want to know which serial interconnect point (JP5 or JP6) is the
RX INPUT (from the point of view of the calculator).

I am planning to duplicate Marcus Von Cube's 2.5mm serial port mod. link
Even though the photograph and instructions say the JP5 is RX I need to know if this RX designation is from the "Calculator's" point of view or the "FTDI Cable's" point of view. Depending on the "point of view" the TX/RX meaning is opposite because the FTDI cable's TX OUTPUT will need to be wired to the Calculator's RX INPUT, and the FTDI cable's RX INPUT will need to be wired to the Calculator's TX OUTPUT. I don't have quite enough information to decide which is which.

My guess from looking at the photograph is that JP5 is the RX INPUT from the calcualtor's point of view, which should be wired to the
TIP if the FTDI cable (TX OUTPUT of the FTDI cable), and JP6 is the TX OUTPUT from the calculator's point of view which should be wired to the RING of the FTDI cable (RX INPUT of the FTDI cable). And of course the shaft (barrel) of the FTDI connector should be wired to battery minus. This guess is based on the assumption that Marcus's 2.5mm jack has the same TX/RX wiring definitions as the standard 3.5mm jack of an FTDI TTL-232R-3V3-AJ cable. Since Marcus said he cut the end off and rewired it, I can't be sure this assumption is correct, however.

If no-one knows, I will open the calculator (ON), attach a scope to each point (JP5 & JP6) and try to sway the signal to ground or +3V with a 5K resistor. The one that "MOVES" is the input and the one that doesn't move is the output.

If someone KNOWS the answer already, then I can skip this procedure.

Thanks in advance to anyone who knows the answer to this question.

Edited: 8 Sept 2013, 10:51 p.m. after one or more responses were posted

There's a bunch of information about the 30b (and smaller brother, the 20b) here:

Wiki 4 HP

If you don't find what you need there, I'm willing to create entry pages on the 30b wiki for what you think would be helpful. Or, by all means, you can create/edit any pages you want yourself, and that would be very much appreciated!



Thanks, I can use the Wiki's internal connector pinout of the 6-Pin programming connector to ohmmeter check weather JP5 or JP6 is wired to the RX INPUT (J35) or TX OUTPUT (J33), to derive the information that I need. I am still waiting on some parts to arrive before I open up the calculator. After I open up the calculator, I will post my findings for others.

Edited: 8 Sept 2013, 10:41 p.m.

Have a look at page 6 of this document

JP5 is RxD and JP6 is TxD (from the calculators point of view)