Double MEM module


Dear HP-41 enthusiasts,

Can someone point me to a direction on how to open a HP-41 memory module and how to combine two modules to a double mem. Many people aren't interested in the simple mem modules anymore but I'd like to free some modules to send the to Diego for NOV64 assembly.

Thanks and Best Regards


Hi Daniel,

Seems that I'm the most appropriate to post a reply... :-)

Frist of all, thanks so much for your interest and contribution.

The Dual Memory modules are built by hardwiring line B4.

Facing the module connector B3 and B4 lines are the lower rightmost, being B4 the inner one.

One of the memory boards must have its B4 line tied to V+ while the other memory board must have it tied to GND.

V+ and GND are in the upper connector row, being GND on the left of B4 and V+ on its right (again facing the module connector).

Regarding the oppening method, I've been using a sharp nose pliers with good results during the past years.

Some PICS may also be heplful:

Insert the nose pliers on one connector slot end.

Apply gentle force to take both halves apart, and work the opposite end in the same way.

Most modules open with minimal or no damage at all.

Hope this helps.



Hi Diego,

Thanks very much for your swift reply with nice pics. Am I right to connect all contacts between the two modules except B3 and B4? Sorry just don't want to fry my HP-41. Btw are you already producing the NOV64? It would make a nice xmas present for myself ;-)

Regards Daniel

Edited: 11 Nov 2008, 1:31 a.m.


Dear Daniel,
a detailled description about creating Double X-Memory modules you find in my new book "HP-41 Input / Output Board & IL2000 System Interface".

Regards - Christoph Klug

Edited: 11 Nov 2008, 2:36 a.m.


... and besides that chapter is also full of other great stuff! [just saw that post after I left mine. Should not do things that late...)


Hi Daniel,

if you want to connect 2 Memory Modules (not X-Memory), you can hardwire the port by cutting the contact of B3 and B4 between the connector and the lower PCB and connecting B3 and B4 to Vcc on each PCB to select the port. It is ok to connect all lines except B3 and B4 (not all are needed).

Contact   12 11 10  9  8  7  6  5  4  3  2  1
Signal B3 V+ B4
Connector | | | | | | | | | | | |
| | | | | | | | | |
Port 1 | | | | | | | | | | | |
| | | | | | | | | |
Port 2 | | | | | | | | |--| | |
| | | | | | | | | |
Port 3 | | | | | | | | | |--| |
| | | | | | | | | |
Port 4 | | | | | | | | |--|--| |

This shows how to hardwire a PCB of a module to a port. "|--|" means: Connect this line on the PCB to V+. If you want to make a double memory module hardwired to port 1 and 2: Connect all contacts beside B3 and B4 from the connector to both PCBs, leave B3 and B4 open on one module and connect B3 to V+ and leave B3 open on the other one.

You dont need to connect B3 or B4 to GND. On the HP41 connector they are left open and the port is selected only by connecting V+.

Double X-Mem are made the same way, but only line B3 has to be different between the two PCBs, as there are only two possible places for X-Memory Modules: left or right.




Hi Diego,

why do you label B4 and B3 different? Christoph Klug and I label the outer one B4.





Yep, I took it the wrong way... (the history of my life... :-(

Althoug it won't make any difference regarding double-memory modules construction, Christoph's article and your post description place B4 and B3 on their correct positions:

GND, B3, V+, B4, Bat.

Sorry for the cofusion.


PS. I'll leave my previous post unchanged to avoid inconsistency on this thread.



I believe Christoph Klug wrote an excellent article for this a few years back for the forum. A quick search there (or on google with double module brings us quickly to this link to his article Hope this helps.




Dear colleagues,

I'm overwhelmed by the response I've gotten on this topic. Thanks very much. I'm now feeling confident to produce a double mem (not xmem).

Regards, Daniel

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