PDP-8 /e/f/m memory

[Allison]

>
>Subject: Re: PDP-8 /e/f/m memory
>   From: Don <THX1138 at dakotacom.net>
>   Date: Mon, 14 Aug 2006 20:42:59 -0700
>     To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
>
>Patrick Finnegan wrote:
>> On Monday 14 August 2006 19:43, Ethan Dicks wrote:
>>> On 8/15/06, woodelf <bfranchuk at jetnet.ab.ca> wrote:
>>>> If you use 2167's ( 16k x 1 ) ram's
>>>> you can have 16k of memory ( 12 chips ) for
>>>> about $25 not counting glue logic and other stuff.
>>> 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'd probably do something like what the SBC-6120 does... use some typical 486 
>> motherboard cache SRAM.   Cheap, and relatively easy to find. :)
>
>Outrageous overkill.  What's that, 10ns memory?  What's the 8's
>memory cycle time??  ;-)

PDP-8 is slow, 1.2 to 1.6us memory cycle time (read, modify, write)
so anything under 500ns is overkill.

Cache rams can cycle fast, those from 486boards were typically
in the 15- 25ns range.  However, overkill is ok as finding a source 
for show rams is unlikely (old 2102s were orignally that slow).
Since most parts I have like the Alliance 7C256 are cmos slowing 
them down also allows for lower power.  Even if they were
NMOS a mere 25ma (I have some old 61256s) standby would still 
allow for short term backup as its only two of them.  Since the
system is not portable (it is a PDP8!) and lives near outlets a 
wall wart of small size could easily supply the power for the 
entire memory rather than resort to battery.

For my case it's not an issue, it's ok if it forgets after power off.
Usually when I demonstrate the PDP-8 I like to toggle in a 20word 
program to do inchworm on the accumulator lamps.  So even if it forgot
wheres the harm.  

I went through the exercise above to point out that with some 
imagination there are solutions.



Allison


>
>> You'd end up with one or two chips to do 32k x 12.
>> 
>> Pat