PDP-8 /e/f/m memory

C. H. Dickman chd_1 at nktelco.net
Mon Aug 14 20:14:34 CDT 2006

Ethan Dicks wrote:
> Why not use a pair of 62256s?  32Kx16 and ignore 4 bits.  It'd even be
> easy to use a Dallas DS1210 or something like it to provide battery
> backup.
I had the same idea back in December of '98.

I was just looking over my notes, which are not as clear as they should 
be, but I'll give a try at converting them to a schematic. Here is a 
summary though:

Two 62256 low power SRAMs with 4 bits ignored. The MA bus is buffered 
and inverted with 74HCT14s. The MD bus transceiver consists of more 
74HCT14s, 74LS240 tri-state buffers and 74LS01s OC drivers, all wired 
together as sort of a DEC005.

In retrospect, the 74LS01s are probably a bit weak for this purpose and 
the 74HCT14s might not have the ideal threshold.

The control logic is trivial. The SRAM /CS is effectively grounded. /OE 
is controlled by MD DIR L. /WE is INHIBIT H NANDed with /OE. Note that 
just like core, the memory is written after every read.

I complicated things a bit because I wanted to have the 3 core fields 
installed and have the rest semiconductor memory so there are switches 
that de-assert /CS when accessing fields 0, 1, or 2. I also have a write 
protect for field 7 so there is a switch that prevents asserting /WE 
when accessing field 7. I used this for storing a boot loader.

The battery backup circuit is bit complicated with lots of 1N914 diodes. 
I was worried about standby leakage. I even used a reed relay to 
interrupt /CS on loss of POWER OK H. It seems to have worked though 
because I am still using the same 3.6V lithium battery after almost 9 years.



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