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16MB SE/30 RAM SIMMs

tt

Well-known member
I ordered some RAM SIMMs and was sent some that are the 3 chip type (after insisting on low-profile SIMMs), and I was wondering if anyone knows if they are ok to use in the long run. I installed them and they boot OK. These are fairly low-profile. In general. there are a few different standard heights for SIMMs, and I believe the maximum height you can use is 0.75" for an SE/30. The 3-chip type are even shorter.

 

Unknown_K

Well-known member
3 chip have parity and 2 chip don't (never seen 2 chip over 4MB). Generally if they work they work, and parity will work on a vintage PC while non parity usually does not (if that matters to you). RAM speed is more of an issue but anything 80ns or less would do fine.

 

tt

Well-known member
JDW, how tall are those? I suspect they are 0.75". I had ones shipped earlier that had two rows of chips, and those were too tall (1").

 

trag

Well-known member
If they're working, then the three chips SIMMs should be fine.

There are occasional reports of three-chip (or two chip) SIMMs not working in old Macs, and I have always wondered about that.

A SIMM is very simple, and a 30 pin SIMM especially so. There are twelve address pins, eight data pins, and a handful of control pins (R/W, RAS, CAS).

The SIMM circuit board connects these pins to the corresponding pins on the memory chips. What could be simpler?

The address and control pins fan out and connect to all of the memory chips. Every memory chip has all the address pins (may be fewer than 12 on lower capacity SIMMs) and the three control pins.

On an eight (or nine) chip SIMM there is one unique data pin for each memory chip. In other words, each memory chip is responsible for storing just one bit out of every byte in memory.

Two chip SIMMs exist because some memory chips store 4 bits at a time, instead of just 1 bit at a time.

On a two chip SIMM four of the data pins go to one memory chip. The other four go to the other memory chip.

There's no logical reason why a 2-chip SIMM shouldn't work anywhere an eight chip SIMM works. "Logical" in the digital sense. There may be an analog reason why a two-chip SIMM won't work.

On a two-chip SIMM, two chips are connected to the address and control pins. On an eight-chip SIMM, eight chips are connected to the address and control pins.

Each chip consumes a little bit of electrical current at the I/O (address/control/data pins) pins when receiving signals. This action is referred to as "the pins sink current". It's a tiny amount, because the ideal is for the I/O pins to just sense the voltage level without consuming any of the current. So designers strive for the ideal, but never quite achieve it.

So a two-chip SIMM sinks about 1/4 the current from the address and control lines as an eight-chip SIMM does.

It is possible that this difference in sunk current causes ringing or other signal problems in some Macintoshes, such that they will not work with some 2-chip SIMMs. The Mac memory control circuits are designed to provide enough current on the address and control lines to drive eight chips per SIMM. If the SIMM doesn't consume enough of that current, it might cause voltage swings.

An interesting experiment, if one ever finds a situation in which 2-chip SIMMs don't work, would be to replace 2-chip SIMMs with 8-chip SIMMs, one at a time, until/if the board starts working again.

If my hypothesis is correct, there should be a point at which replacing 2-chip SIMMs with 8-chip SIMMs results in a high enough sunk current that the board would start working again, even though some 2-chip SIMMs are still installed.

 

Charlieman

Well-known member
Trag

There's no logical reason why a 2-chip SIMM shouldn't work anywhere an eight chip SIMM works. "Logical" in the digital sense. There may be an analog reason why a two-chip SIMM won't work.
I've used two and three chip 32 pin SIMMs from c. 1990 in Macs with varying success. For obvious reasons, I used 1MB SIMMs when testing 68000 compacts.

The LC and LCII seemed happy with two and three chip 32 pin SIMMs of different sizes.

The Plus and SEs that I tested hated two and three chip 32 pin 1MB SIMMs. They ran happily on eight or nine chip SIMMs.

It is a case of "what works". I would not presume that an SE/30 works with two or three chip SIMMs because the technology did not exist when the SE/30 was designed. I would *expect* that an SE/30 works with suitable eight or nine chip SIMMs.

 

zuiko21

Well-known member
The SE/30 works fine with 2/3 chip SIMMs -- just checked.

Assuming 1 MByte size SIMMs, "regular" 8-chip modules use eight 1 Mbit DRAMs -- with another extra chip for parity (9 chip) modules, which simply goes unused in most Macs.

But those SIMMs with 2/3 chips use a couple 4 Mbit DRAMs (1 M x 4 bit) with an optional 1 Mbit DRAM (1 M x 1) for the parity bit, if present.

4 Mbit and larger DRAMs include a new test mode which is indeed incompatible with the circuitry of the Mac II & IIx -- that's why they need PAL SIMMs for larger capacities. These 2/3 chip SIMMs aren't compatible either. Maybe the SEs & Pluses (designed for SIMMs no larger than 1 MByte) have similar issues.

The SE/30 will work with any (fast enough) 30-pin SIMMs, be them 2, 3, 8 or 9 chip, as long as they aren't of "odd" sizes -- 2 MB and 8 MB SIMMs are not supported.

 
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