WP 34S - best cable layout ?

[burkhard]

(01-26-2021 01:34 PM)rallyrabbit Wrote:  Also found these two threads:
https://www.hpmuseum.org/forum/thread-30...hlight=30b

This link doesn't apply unless you have one of the Harald boards installed. Those are not required, but permitted flashing via USB.

Quote:https://www.hpmuseum.org/forum/thread-24...hlight=30b
What I can gather is the we have to ensure the Serial to TTL is not using 5V for sure for Vcc, and that the TX/RX lines are also 0-3.3v rather than 0-5v. Everything else appears to be optional.

Just do it like the Theremin guy did. The two 3V coin cells on the calculator are in parallel (for longer battery life). Remove one of the cells. Install a fresh one on the other side (you don't want low volts when flashing).

Here are some notes I took (or probably copied from his original web page):

"The two coin batteries are actually wired in parallel, so I just pulled one out to get at +3V and ground. The then exposed center battery conductor is ground, the rim conductor (located above the center connector, up inside the housing) is +3V.

You have to erase the firmware, then program it with the serial cable. Erasing is performed by power cycling and resetting the calculator with the lower left serial port hole connected to +3V (there is a small "reset" hole with a switch behind it located directly above the serial port).

Those are 1k resistors in series with the serial leads to limit current just in case (battery voltage is ~3V, the TTL TX level is ~3.3V). On the serial port: green=TX (data into the calc), white=RX (data out of the calc), red=+5V (unused for this project), black=ground (common for TX & RX).

Serial programming took all of maybe 20 seconds, and it's not too hard to keep the connections steady for that long, even hand-held as in the picture. If you botch the programming just start over, I don't think there is much - if any - risk of bricking it. Don't leave the batteries in it unprogrammed, otherwise they might get quickly drained."


Me back again. Please note once more: The red wire in the cable is not needed for either the erase procedure OR the actual reflash. You don't use it. Just follow the picture on the ThereminWorld archive page where he is holding the resistor leads to the TX and RX pads and the black wire (ground) is hooked to the ground terminal from the removed coin cell. I don't think you actually need to use the +3V from the open coin cell socket because that is only used to trigger the ERASE and there is a pad supplying 3V.

Wire Summary:
Red: not used
Green: TX -- hold to top left flash pad through a 1KΩ current limiting resistor
White: RX -- hold to top middle flash pad through a 1KΩ current limiting resistor
Black: alligator clip to the ground terminal (in center...looks like a 3-pointed spur) of open coin cell cavity

Attached is a schematic of the flashing pins on the back of the calculator and a flashing sequence. This I didn't create, merely screenshotted off the original ThereminWorld site a few years ago and saved in my records. It seems to now be missing in the archive of the ThereminWorld page, which is a pity.