HP-71B under the scope.



Hp-71B bus signals after a cold start.
Traces (from the bottom) are Command, Strobe, and B0-3.
This is the first command set after a cold boot.

After confirming the protocol we hope to build a flash based module and eventually a USB module emulator.


I'll take one of each. :-)


Put my name on the list !!!



Good for a start. I did more or less the same when starting my MLDL2000 project for the HP41. Do not hesitate to contact me if you want some details on how I did it ..



Thanks for the kind offer. Your kit looks marvellous!
I think my job is easier then yours. I have a full hardware spec and it seems the 71B bus is easier to work with than the 41.
The min time between bits is 1.6 uS - 625 Kb/s. The serialisation has me worried, it might go as high as 5 Mbit with 4 data lines and 4 control lines.

My sample unit has been a troublesome little beast from day one.
First it was DOA from the eBay seller (I could say more).
Then I noticed it drawing 2 mA - turned OFF!
Naturally any loss of battery resulted in reset. Not that the batteries lasted very long anyway.
Tonight I bit the bullet and opened it up again.
I checked the regulating circuit zener and transistor etc and resistance to ground. Everything looked OK.

I powered up the 'half' 71B and took 'live' measurements, it still looked OK.
The only thing I found was the metallic 'zebra strip' still looked damaged on one end so
I very gently teased the conductors into a semblance of order and cleaned some
corrosion or crud from between some PCB traces.

That seems to have fixed it? The resting current is .06 mA and changing out the batteries is now fine. (as long as you don't press a key!).
It's survived for over a minute with no batteries so the 'change out' cap is fine.

I hope the little beast is settled as this job is hard enough without an unreliable control unit.

peter walker



With today's electronics the speed is not going to be an issue. You will probably be doing serial to parallel conversions anyway. My little toy could potentially run at a few MHZ, compared to the current HP41 clock at less than 400 kHz.
What I have seen with USB is that the USB speed is OK, but the latency is too high to make a reliable emulator that runs on the PC.
I would be surprised if you could not make use of the same hardware platform as I am using.


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