torq@tardis.computer-science.edinburgh.ac.uk (06/04/91)
I am hoping to design a 4 meg ram card for my 540, as the acorn ram card seems
ridiculously overpriced at 449 quid or so.
My question is :-
What does IC39 do to page the other slave MEMCs? As far as I can see it uses
the unused bits in the page descriptor to page each MEMC but how?
I do have a copy of the VLSI manual of course, but there doesnt seem to be any
pointers in there. I've calculated that I can make a ram card for approx 150
quid, so it does seem the acorn card is ?slightly? overpriced
Has anyone else looked into this subject?
Any help or pointers appreciated.
Andy
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Andy Mell (torq@uk.ac.ed.cs.tardis)
(torq@gnu.ai.mit.edu)nbvs@cl.cam.ac.uk (Nicko van Someren) (06/05/91)
In article <9106041851.AA18049@ucbvax.Berkeley.EDU> torq@tardis.computer-science.edinburgh.ac.uk writes: >I am hoping to design a 4 meg ram card for my 540, as the acorn ram card seems >ridiculously overpriced at 449 quid or so. > >My question is :- >What does IC39 do to page the other slave MEMCs? As far as I can see it uses >the unused bits in the page descriptor to page each MEMC but how? > In the 540 as I recall a PAL takes A7,A8,A22 and A23 in and bodges versions of these A7 and A22 for each MEMC. I think it takes in a few more than this infact. Each MEMC can either be a master or a slave and as such resonds to different A7 levels for updateing the CAM tables and different A22 levels for access to phisical memory. If either ther CAM or phisical memory is being access the PAL fixes these siganls so that only one MEMC is active. To make a RAM card you don't need to know any of this, you just wire up the RAM to a MEMC and the conector and go. > I've calculated that I can make a ram card for approx 150 >quid, so it does seem the acorn card is ?slightly? overpriced Take a good look at the RAM cards Acorn sell. The system runs at 12MHz, there are two large connectors between the RAM and the processor. 12MHz has a cycle time of 83ns, lose say 5ns on each conector, a few more for track capacitance, you need beyyer than 70ns RAM on the card. Acorn use 60ns RAM on the ones that I have seen. So the RAM will cost, on a good day, 130quid. 15 for a MEMC (if you are Acorn), a gold plated 96way DIN connector (3 quid), a single 7400, some decoupling caps. and 10 square inches of 4 layer PCB. So if you get the parts at Acorn's prices you could maybe do it for 165 including construction. Now consider that Acorn sell to the distributer, the distributer sells to the dealer and the dealer sells to you. So in short, No, I don't think it is over priced. Nicko +-----------------------------------------------------------------------------+ | Nicko van Someren, nbvs@cl.cam.ac.uk, (44) 223 358707 or (44) 860 498903 | +-----------------------------------------------------------------------------+