How to disable additional RAM on a 486SLC CPU based system? - Page 1
Forum section: electronics / projects

author: caius
author_rank: Regular Contributor
timestamp: April 26, 2023, 10:08:52 pm
content: Hi all!I designed a 2MB RAM expansion card for an old computer which is powered by a 486SLC CPU (QFP100 package).The card is working fine and I'm happy with it.Now I woud like to implement the possibility to disable this additional RAM making it invisible to the whole system.Which is the correct design in order to achieve this?Perhaps by putting tri-state the data bus of the expansion card?Here you can see the 486SCL datasheet :http://www.bitsavers.org/components/cyrix/Cyrix_Cx486SLC_Data_Sheet_Feb92.pdfThanks in advance for any advice or help!

author: james_s
author_rank: Super Contributor
timestamp: April 26, 2023, 11:50:09 pm
content: Can't you just disable the address decoder so it stops asserting the chip enables for the RAM ICs?

author: caius
author_rank: Regular Contributor
timestamp: April 27, 2023, 07:18:09 am
content: Quote from: james_s on April 26, 2023, 11:50:09 pmIf I remember correctly I tried to tie to VCC the /CE pins of the RAM chips but it did not work.Anyway, here is attached the address decoder.

author: james_s
author_rank: Super Contributor
timestamp: April 27, 2023, 10:31:58 pm
content: Well if the OE on all of the RAM chips is disabled then they won't be putting anything on the bus, so I'd think that would be the same as removing them. I haven't seen the rest of your circuit though.

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 28, 2023, 02:10:00 am
content: What type of RAM do you have? If the motherboard and memory use serial presence detect, then it'll probably still think the RAM is there even if you disable the /CE line.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: April 28, 2023, 03:31:30 am
content: I'm pretty sure SPD wasn't a thing in the 486 era. IIRC it only came about with the introduction of DIMMs, and didn't exist for the SIMMs of 486 and earlier platforms. And it sounds like OP is using neither interface anyway.

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 28, 2023, 03:48:42 am
content: Maybe not... Might have been a parallel interface for the 72pin SIMMs back in those day.Either way, the OP should post a schematic of his circuit rather than us playing the guessing game.

author: caius
author_rank: Regular Contributor
timestamp: April 28, 2023, 09:28:39 am
content: Sorry for late reply.They are 4Mbit parallel RAMs :https://www.mouser.it/ProductDetail/913-AS6C4008-55SINIIRC I already tried the disable the /CE lines and additional RAM was still recognized by the system.Attached are schematics.

author: Kim Christensen
author_rank: Super Contributor
timestamp: April 28, 2023, 04:47:19 pm
content: Quote from: caius on April 26, 2023, 10:08:52 pmCan you show us a complete schematic of this RAM expansion card? Does it insert into a standard interface on the PC motherboard? What's the model/name of this PC and interface?

author: caius
author_rank: Regular Contributor
timestamp: April 28, 2023, 05:12:59 pm
content: Quote from: Kim Christensen on April 28, 2023, 04:47:19 pmThe RAM expansion card plugs into a rear port connector (SCSI-80) of a japanese computer of '90.Since it's a commercial product I cannot share schematics, I hope you can understand.

author: ejeffrey
author_rank: Super Contributor
timestamp: April 28, 2023, 05:53:07 pm
content: Quote from: caius on April 28, 2023, 09:28:39 amIf the /CE is pulled high, the data pins will be high-Z and it won't respond to WE / OE controls. If that isn't working, it means that either you haven't actually disconnected and pulled high the /CE pin, or the motherboard is using some other sense mechanism to detect the expansion card rather than the RAM chips themselves. This could be (for instance) in presence detect pins that are shorted to Vdd or GND right at the 80 pin connector, or even the connector itself having a physical presence switch similar to how TRS and DC barrel jacks often work.Can you describe what happens when the "additional RAM is still recognized"? Does POST fail because it detects memory that is disabled?

author: caius
author_rank: Regular Contributor
timestamp: April 28, 2023, 08:39:37 pm
content: Quote from: ejeffrey on April 28, 2023, 05:53:07 pmWell, I just tried again to tie all the /CE pins to VCC via pull-up resistors and system does not boot at all, it saty stuck on POST screen.I can exclude there is some physical switch on the SCSI-80 connector, at this point I think there is some signal that must handled along with /CE tied high in order to totally disable the additional RAM on the card.EDIt :Actually the system has a 386SX on board but it can be upgraded with a 486SLC (anyway the latter is fully compatible with the first)