The PC-1500 is a really well designed pocket computer and offers many ways to extend it’s software and hardware capabilities.
Some companies were seduced by the Sharp solution and it’s “owner” memory modules.
EDTech was one of these companies and were offering a solution for education administrators :
(Picture taken from eBay)
Externally it looks like the PC-1500A + CE-150.
But internally there is a custom memory module :
It holds a 57256 32KB EPROM
We have to distinguish three kinds of memory modules :
– RAMs stored between &0000 to &3FFF (like 4KB CE-151, 8KB CE-155, 8KB CE-159, 16KB CE-161 or 2x16KB CE-163)
– ROMs or EProms stored between &8000 to &BFFF (that extends the system with Basic instructions)
– Pseudo ROMs stored between &0000 to &3FFF (like the 7,8KB CE-160 used to store custom invisible Basic programs)
– EProms stored between &0000 to &3FFF used to store invisible Basic programs
The EDTech ROM is in this last case.
But if you understand address decoding “&0000 to &3FFF” is only 16KB wide.
On this module there is a TTL 74LS73 IC. It’s a dual J-K flip-flop with clear.
It’s function is to select one of the two 16KB pages.
Open the Technical Reference Manual and look for memory configuration and /S3 signal.
On the PC-1500 this signal select a memory zone starting at &5800.
On the PC-150A this signal select a memory zone starting at &6800.
Nice, this can work with the two pocket computers.
On my schematics you can see it connected to the CLK pin of the Flip-flop.
Now examine the content of each 16KB page (only the beginning, the rest is the EDTech program)… lines 11, 40 and 41
1st page :
2nd page :
The POKE instruction writes a value to a memory case. By this way the program can swap the page.
Depending on the address it writes a 0 or 1 on the J and K entries and the clock push it to the Q and /Q outputs.
It’s a very nice design, isn’t it ?
I asked EDTech to get some informations and maybe a user manual.
They answered but nobody was able to help me.
Can you ?
Contact me if you have other “non Sharp” modules… 😉
Yes, this was the second interesting thing in this module 😉
They used same connector as in the PC-1500 to program the EProm and didn’t have to scramble anything.
But if you unsold the EProm and try to read it with another system you will get something that you can’t disassemble.
Wow! That is really interesting!
I also noticed that the address lines to the EEPROM are scrambled — bit A0 is toggled rather than the MSB. It must have made the PC Board layout easier – they must add a scramble step before they program it.