Convert HP38C to HP34C, is it that simple?

[Marcovecchio]

I would like to have a 34C for a long time, but prices are too high lately. However, I have a financial 38C, in bad physical shape, but working fine. Then I wondered: what is the difference between both models? Can I convert the 38C into a 34C?

While researching, I found the excellent Eric Smith's IC listing for all HP calcs, (http://www.brouhaha.com/~eric/hpcalc/chips/), and it says 38C and 34C use the same processor! My interpretation it that the only difference is the ROM chip!

And, to make things even better, I have a 34C faceplate and a full set of keys, to make it looks exactly like a 34C.

So, my question is: is the ROM chip the only difference?

Thanks!

Eduardo

[tangdfx]

Panamatik the low power spice will be the final solution to your problem.

[GreyUser]

It’s more than just the processor, you’d need all three of the ram/roms chips from a 34C to convert a 38.

[Marcovecchio]

Thanks for the replies!. Since I prefer the original hardware, I'll keep an eye open for a damaged / for parts 34c to salvage the ROMs. But thanks for the Panamatik's option, I could go this way if the original plan fails.

Eduardo

[teenix]

Both calculators have ROM space from $0000 to $1FFF which is 8K.

There are only 3 ROMs shown on the circuit board with 2K each so the processor package has a 2K ROM chip inside.

For the 34C and the 38C there is 2K of ROM that is the same, from $0000 to $03FF and from $1000 to $13FF, so this is most likely inside the processor IC and why the 2 models have the same serial number.

The 2 models are C types, so if you have the ROM IC set for the 34C then I would guess that it would work.

It is interesting that the addressing is split this way and is probably the same in the other 3 ROMs. I suspect it is because of the ROM checksum function that occurs by pressing [STO][ENTER].

Also, for every BANK SWITCH instruction in each 4K bank of ROM, there is a corresponding one in the other bank otherwise this checksum function would not work.

cheers

Tony

[tangdfx]

Hello, Tony

In the service manual said that unlike the ACT of previous calculators, the
CPU contains 1024 words of ROM. This ROM is the
only section of the CPU that differs between the three
CPU versions.

In all, the CPU consists of 11 basic sections:
a. Timing and control.
b. Address, status, and format register.
Pointer and status counters. C
d. Instruction buffer and decoding.
e. Data registers.
f. Arithmetic and logic unit.
g. Self-check logic.
h. Cathode driver logic.
1. Anode driver logic.
Keyboard interface.
k. ROM and associated logic.

[teenix]

Interesting, I would have thought 2048 words.

Is the service manual available for download?

cheers

Tony

[Steve Simpkin]

(05-19-2022 05:03 AM)teenix Wrote:  Interesting, I would have thought 2048 words.

Is the service manual available for download?

cheers

Tony

The Series E Calculators Service Manual (March 1978) is available at the link below. It does not cover the HP-34C which came out later. It only describes the press-fit construction used on the early production runs.

https://literature.hpcalc.org/community/...s-e-sm.pdf

[tangdfx]

Hi ， tony

I assume you already know about that.

You can refer to the site: literature.hpcalc.org

Under the model section HP33E there is a file named "HP Series E Calculators Service Manual"

Although this is related to E model. But there is no big difference between C and E model except the PSU module which supply the power to one ram in C model.