HP25 RAM

[Alexanderpentagon]

Hi , I'm new in this forum , Thank You for my admission.
I have two HP25 , one of two has ACT chip ok but RAM broken.. I would to fix the issue with a new circuit that emulate the function of the RAM . I'm collecting some information about the electric connection and logic operation of that chip.
I have found this:
https://www.hpmuseum.org/cgi-sys/cgiwrap...read=83799
but I need to know if there are some other info about RAM related instruction in order to made a microcontroller/cpld surrogate of that RAM.
Do You have some link or threads about that?
Thank You in advance
Alex

[Dave Frederickson]

I recommend the ACT Repair Kit for the HP-25.

http://panamatik.de/html/hp_calculator_repair_kit.html

[teenix]

As Tony Duell mentioned in the Forum article, you will have to implement serial data flow for instruction decoding to read and write RAM.

Clear Data Registers
c -> data address
c -> data
data -> C
c -> data register n (Where n can be RAM address 0 - 15)
data register n -> c (Where n can be RAM address 0 - 15)

These all have to be structured inside the 56 bit instruction cycles. (around 320uS)
The data bus can be driven by the ARC or RAM chips
The C register is normally active on the data bus unless a RAM read takes place
You will have to interface the new circuit to work in a 6 volt environment. A PIC processor for example has a maximum voltage rating which (sort of) allows 6V operation but is not recommended to operate above 5.5V. To avoid throwing the dice of chance, a better choice would be a high voltage tolerant input type or creating an interface.

There is some info on instruction flow in my Classic Notes document.

http://teenix.org/ClassicNotes.pdf

cheers

Tony

[dmh]

I have a 25 with RAM not working and someone on the forum suggested reflowing the solder on the RAM chip (haven't tried).

The 25 is pretty common too so another RAM chip from a donor is probably a good option too.

[Alexanderpentagon]

Thank You!
I suppose there are not changes from the classic calc to the woodstock
Now I can start to project !
I'll make a level translator or use LTC1045.....
KR
Alex

(09-02-2021 01:42 AM)teenix Wrote:  As Tony Duell mentioned in the Forum article, you will have to implement serial data flow for instruction decoding to read and write RAM.

Clear Data Registers
c -> data address
c -> data
data -> C
c -> data register n (Where n can be RAM address 0 - 15)
data register n -> c (Where n can be RAM address 0 - 15)

These all have to be structured inside the 56 bit instruction cycles. (around 320uS)
The data bus can be driven by the ARC or RAM chips
The C register is normally active on the data bus unless a RAM read takes place
You will have to interface the new circuit to work in a 6 volt environment. A PIC processor for example has a maximum voltage rating which (sort of) allows 6V operation but is not recommended to operate above 5.5V. To avoid throwing the dice of chance, a better choice would be a high voltage tolerant input type or creating an interface.

There is some info on instruction flow in my Classic Notes document.

http://teenix.org/ClassicNotes.pdf

cheers

Tony

[Alexanderpentagon]

Yes , I have tried to change but I have no results... with the second ram I have a strange result of 40 read in all of the memory program step... but I suppose that there are not perfect contact between chip and socket ( one row of pin is cutted and there aren't a firm stop of chip in socket) I'll try to solder the chip by wire for verify that the chip is broken and then I'll order a ultra low profile socket ...
Alex

(09-02-2021 03:42 AM)dmh Wrote:  I have a 25 with RAM not working and someone on the forum suggested reflowing the solder on the RAM chip (haven't tried).

The 25 is pretty common too so another RAM chip from a donor is probably a good option too.