My WP-34S : USB to Serial cable prototype

[jebem]

(04-12-2014 06:40 AM)olr Wrote:  

• You are connecting 3.3V to Vcc of the calculator. This is probably neccessary when the batteries are removed. But what if not?
• The RESET pin is already high(3V). I think a reset of the calculator is triggered when the RESET pin is connected with GND?
• My USB-TTL converter is also based on Prolific PL2303HX and TX/RX are operating at 3.3V and it has an 3.3V output pin, that might be used.
  

Usually FTDI chips are recommended. However i flashed my HP20b successfully with a PL2303HX-based converter.
olr

Hi Olr,
Thanks for your comments!
Well, this is just a prototype, not the final version... it is really a work in progress. And I never flashed a HP-30B before.

As for your excellent remarks:

- I expect to use the external 3.3VDC to supply the calculator as well, removing the cell batteries first. Of course none of this is needed, one can just use the cell energy to do all the operations, including the Erase function as others have reported here in the MoHPC.
But I do not know how much current the Erase and flash operations can consume and I do not want to deplete cell batteries one after another.
As I said, I am waiting for the pogo connectors and then I will be able to do some readings.
But I have made some current readings on my HP-30B and it can consume in excess of 20mA, and that is a lot for cell batteries (!), for instance when one runs the diagnostics (ON + PMT, then scroll down to "TSys" setting and change it from "Off" to "On"...).

- As for the RESET: Good point! I have just checked (again) my own HP-30B, and You are right, in normal conditions the RST connector has +3VDC.
When I did my previous readings it was at Zero Volts, so I assumed the RST was working in the same way as the ERASE.
In fact, if one press the RST button in the back, the RST connector voltage goes down to Zero Volts (as expected) but then it stays a Zero Volts! The only way to restore the normal condition is to Power On the calculator (or remove the batteries). That was probably why I was reading Zero Volts initially.
I will correct my diagram and publish it in the initial post.


- As for your last point: The Prolific PL2303HX having an 3.3V output.
Yes, I saw that possibility as well, however I checked the datasheets and apparently the current supply available on this 3.3V supply is really low.
This 3.3V supply (Pin 17 - VO-33) typically consumes up to 20mA (as per RS-232 standards?), so probably can supply that much, but it already has to supply current to the TX serial line, so I do not want to take my chances overloading it with additional Erase duties.

- And many Thanks for sharing your experience with this Prolific IC, instead of the more "recommended" FTDI one.

Now, I cannot wait for those pogo pins...