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DECpc LPv+ 486 Ideal Cache Speed and Type

iMic

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Joined
Dec 15, 2016
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Location
Adelaide, Australia
Hi folks,

I'm reassembling a rather good condition Digital Equipment DECpc LPv+ at the moment. It was originally a 486 DX-33, but it's now running a DX2-66, which is about as far forward as I'll be going with the processor. Really the machine is meant to be a hobby system for DOS to low-level Windows 95 at the most, so it doesn't need to be absolutely maxed out.

I need to sort out the cache situation though. The system has 3x DIP-24 sockets for Cache Tag, and 8x DIP-28 for Cache. According to the hardware maintenance manual for the LPv+, the machine requires:

  • four or eight 32KB x 8 cache SRAM chips
  • three 16KB x 4 cache tag SRAM chips

One example of a working board I saw used three Cypress Semiconductor CY7C166-15PC 15ns chips for Tag, and eight Cypress CY7C199-20PC 20ns chips for Cache. This seems like a safe bet, since it's proven that this configuration and part works in this board, but I'd like some input from those more knowledgeable before I commit to ordering them.

Is 15ns Tag / 20ns Cache ideal for a 486 DX2-66 on a 33MHz bus? Should the chips be quicker, or is there no benefit in this configuration and substituting a full set of 20 or 25ns chips (which are sometimes more readily available and a bit cheaper) would also suffice?

I should add that the ICs don't need to be Cypress - Winbond, ISSI, Mosel-Vitelic, Motorola, etc works as well. But I've only been able to find part numbers for Cypress and Motorola in the DIP-24 16K x 4 ICs, and only Cypress, Winbond and Mosel-Vitelic for the 32K x 8 DIP-28's. Since I have Cypress part numbers for both, I figured purely for "lid-off aesthetics" I'd go for a matched set.
 
The period of 33MHz is 30ns. From that tidbit of info, I say 20ns RAM ought to be good enough, however others may know about potential timing nuances of the 486 that I don't.
 
For 33 MHz, you typically use 20s for Cache and 15ns for Tag. So yes, the example you saw is correct.
 
20s is fine, up to a dx2 66., if using 20ns for write through cache. 15ns if write back. After that stuck with 15ns or faster. Some say 10ns for dx4-100 and above. Faster tag is just silly as it works at same speed. Only time you need unobtanium cache chips is if going over 33mhz bus, at least in my experience. 15ns on my 5x86 amd @ 160. Just make sure to buy name brand chips, as in this era was alot of shenanigans with cache chips.
 
I settled on the Cypress CY7C199-20PC 20ns cache ICs. Another spare DECpc LPv board I had already had four installed, so it made it much cheaper to just purchase the remaining four (ended up getting a reasonable deal on 6, so I'll have a couple of spares) and then sort out the Tag.

Sounds like the Cypress CY7C166-20PC 20ns ICs should be suitable for the Tag as well, and those aren't too difficult to source... might be taking a gamble as those are coming out of suppliers in China, but the seller at least seems reputable after some searching around.

The hardware maintenance manual suggests the cache runs in write-back mode, but the 20ns should do fine. Not chasing every last possible ounce of performance out of this system, just making it a pleasant old machine to use.
 
Cypress made great cache chips so you should be good. (y) 20ns for the tag is fine if not going past 40mhz bus, if you try to overclock that is...
 
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