As I managed to scrounge up some time, I decided I would document some of the bits and pieces I’ve managed to collect throughout the years. This post will be dedicated to the SIMMs in my collection – and no, I don’t mean the game!
Single In-Line Memory Modules (or SIMMs for short) were a type of memory used in late 286’s, most 386’s, practically all 486’s and even some Pentium/Pentium II class machines (often as a backwards-compatibility measure). They followed the Single In-Line Pin Package (SIPP) memory, which was very similar with the exception of the use of fragile connection pins rather than pads, which would have been very prone to damage.
The SIMMs were succeeded by Dual In-Line Memory Modules (or DIMMs for short), which persists to this day. Which begs the question – what is the difference?
As it turns out, the SIMMs are named “single” for the reason that their contact pins are in a single row. Despite the contact pads being available on the front and the back, both pads are redundant and interconnected. Thus the two variants of SIMMs (30-pin, and 72-pin) have twice as many pads as pins. SIMMs were mounted in most boards in slots that required inserting the module at an angle, and then raising it up to vertical where it “clips in”. Depending on the processor and chipset, 30-pin SIMMs often had to be installed in pairs or (more safely) in a set of four, whereas 72-pin SIMMs could be installed singly (or later, for Pentium class machines, in pairs). SIMMs themselves were built on two different types of memory, the earlier and slower variant being known as Fast Page Memory (FPM), and the later, faster variant known as Extended Data-Out (EDO). EDO memory offered noticeable performance benefits, mainly by extending the time the data output would be valid for, thus allowing for another memory operation to be queued while the data output of the previous operation was being read out.
DIMMs on the other hand, use the contact pads on each side separately, thus allowing many more edge connections to be made in a given space. Aside from that, they also have a data bus twice as wide as the 72-pin SIMMs, and featured a host of new capabilities including serial presence detect (to identify a module’s manufacturer, capacity and timing). It also featured a larger board, smaller contacts, and the “straight down” mounting system. Most often, these could be installed singly, as they provided a wide-enough bus to satisfy the CPU. The DIMMs started off with the SDRAM series, at 66Mhz, 100Mhz and then 133Mhz and progressed through the DDR, DDR2, and DDR3 era.
Nowadays, SIMMs are a rare sight, and many present-day computer builders may have never seen one. Vintage computer owners and enthusiasts can end up paying a fairly big amount for some large capacity SIMMs to keep their computers and synthesizer cards running. In my youth, I had both salvaged SIMMs from ex-corporate machines, but also been guilty of disposing them as worthless. But lately, I have been on a bit of a “vintage” muse, so I’ve vowed to collect any that happen to “fall” my way.
If there’s anything that can make you appreciate the progress of computing technology, it is to look back. Hopefully you will enjoy my collection of SIMMs – this notably excludes any SIMMs within my still-working vintage workhorses for obvious reasons. If you ever need to work out the capacity of a SIMM, this particular page has a good list of part numbers and capacities – very handy indeed.
The first computer I used as a youth had 30-pin SIMMs. They were 1Mb chips, made by Micron Technology, but unfortunately I had never saved any of them. Instead, here are the ones that are still left in my collection.
This one is a Samsung branded module from 1993, made by Samsung with Samsung Korea chips. The main memory chips are numbered KM44C1000BJ-7 (which are 1Mbit x 4), with the parity chip labelled KM41C1000CJ-7 (which is 1Mbit x 1).
As a result, it is a 1 Megabyte stick of RAM – i.e. 1Mbit x 4 x 2 = 1Mb. It also has an extra bit for parity – i.e. a 1Mbit x 9 configuration. Each of the chips has a ceramic surface mount bypass capacitor mounted close to it – many of these chips easily ate 120mA in operation! Most boards would have had eight 30-pin slots, and these modules would need to be installed in pairs, or more commonly, a set of four.
This is a Mitsubishi branded module from 1993 using Mitsubishi chips. It appears the module was made in USA. The chips are marked M5M44400AJ-7 (1Mbit x 4) and M5M41000BJ-7. This is another 1Mb stick of ram.
This seems to be a much later (1994) “cheap” SIMM. You can tell by the reduced height of the PCB to the bare minimum height, and no real branding. The memory is made of two Motorola MCM44400GN70 with a Texas Instruments TMS4C102DJ-70, making this another 1Mb stick of RAM.
These sized modules were quite common, and I would come across them a lot. It wasn’t particularly exciting, as on an 8 slot computer, that would mean 8Mb tops. Instead, we would salivate over anything bigger. Apparently they even came in capacities as small as 256kB, so I guess having 1Mb is lucky in comparison.
It’s interesting to note that not all systems required parity – it’s presence generally didn’t hurt anything. The suffix on the memory -5, -6, -7 or -50, -60, -70 were a way of denoting the speed of the chips in terms of the RAS access time, meaning 50ns, 60ns, and 70ns respectively. This was a convention followed by most memory chips, and faster chips generally posed no issues, however using slower chips than required could cause issues.
Speaking of larger capacity – this claims to be a 4Mb module. It seems to be sold or distributed through PCDirect, and has an Integrated Memory Systems label on the front (likely, the vendor). The front of the PCB is coded with GT3009. The rear of the PCB has PTC marked on it, and a whole host of horizontal bus traces – it’s pretty unique.
The module itself is made of 9 Micron Technology (think Crucial) chips, each marked with 4C1004JDJ-6 which are 4Mbit x 1 chips. This makes it a 4Mbit x 9 configuration – i.e. a 4 megabyte module with parity.
Larger 30-pin SIMMs were probably available, but rarely seen or used. The number of address lines in 30-pin SIMMs meant that there is a limit to 16MiB addressable. In the case of a regular PC that requires four sticks, the 4Mb modules would reach that. However, in printers and other devices with fewer slots, larger SIMMs may have been used.
It was really a bit of a compatibility issue, as it highly depended on the host system whether it could address all the available RAM installed, and BIOS upgrades were a chip-swap in that era (being EPROMs).
Late in the 486 era, and into the initial Pentium era, 72-pin SIMMs were king for simplicity. No longer do you have to take care matching sets of SIMMs as much as before. Unfortunately, most boards only featured four 72-pin slots, but the modules were often larger in capacity (and double-sided too) which made up for it. If memory serves me right, even 16Mb 72-pin SIMMs were easily found, and 32Mb were rarer but still available (I never owned any though).
If anything, this one 72-pin SIMM is the most hilarious and interesting example in my collection. I salvaged this one from an ex-corporate system when I was back in Primary school, thinking I had hit the jackpot. A module of this size with this many chips would have been quite expensive, or so I thought. The back was even filled with traces. The PCB silkscreen coating suggests it may date back from 1991, but it’s uncertain.
Instead, it has eight Samsung KM44C256BJ-8s and four Samsung KM41C256J-8’s (all 80ns). The datasheets aren’t available anymore, but some quick decyphering of the code suggests it is eight 256kbit x 4 chips, with four 256kbit x 1 chips. This makes 1Mbit x 9 or in other words a one megabyte with parity. It’s hilarious to think this big thing is the same capacity as the small 30-pin SIMM above, but it may have been one of the ways to build a low-capacity 72-pin SIMM while keeping the memory bus width requirement in check.
It’s like welding together four 256kB 30-pin sticks (three chips on each – two 256kBit x 4’s with a 256kbit x 1 parity). In fact, there were SIMM adapters back then which would allow you to adapt 30-pin SIMMs together to make a 72-pin SIMM but I have never used one myself.
That being said, this was the smallest 72-pin SIMM (memory capacity wise) I had met.
72-pin SIMMs also came in some oddball configurations, and were able to go dual-sided as well. In many boards, there was an un-even number of chips on the front and back, which seems to be a case of 8-chips (memory) on the front, with four-chips (parity) on the back. In this case, it seems to be made of eight Vanguard VG264400BJ chips (70ns). These are claimed to be 256kbit x 4 x 4 configuration in FPM, which means 512kBytes per chip * 8 = a 4Mb module it seems with no parity (many systems didn’t need it anyway). I don’t know what that 8Mb scrawling was on the back … It matches up when you see the L431B4MB PCB marking, implying it’s a 4Mb module.
A very common capacity at the time, I know I had a few 4Mb modules in my possession in the past.
This is a later module, a Hitachi built (note the PCB logo) module with all Hitachi chips. It’s model number HB56D13 6BV-7A and is marked 3rd week, 1994, sold 19th August 1994.
The front of the module is adorned with eight HM514400AS7 chips (1Mbit x 4), dated week 47 of 1993. The rear is adorned with four HM511000AJP6 (1Mbit x 1) chips, dated Week 44 of 1992. All of them were made in Korea. As a result, the capacity is 4 Megabytes with parity.
This one also seems to have its presence detect jumpers (surface mounted, in blue) fitted for the computer to use when detecting the presence and capacity of a given module (pre-dating the SPD EEPROM feature of DIMMs).
As time went on, cheaper modules arrived on the market with very few components – notice how there’s virtually no capacitors on this one, with a minimized PCB footprint. This one was dated 15th May 1998 which is a little late in the SIMM game. This one is a double-sided PCB, with space for eight chips on each side (implying no-parity, another money saving measure).
This particular module also seems to have pads for the presence detect feature, but no jumpers (or 0-ohm resistors) were installed, thus it it is not active. It is made of eight Vanguard VG264400BJ (60ns FPM) chips, which is 256kbit x 4 x 4. This makes it 512kB per chip, for a total of 4 Megabytes per module without parity. Using the same PCB and populating both sides, it seems probable that this could have been 8Mb without parity as well.
Different manufacturers were also in the SIMM memory package market – this one is based on Fairchild chips. The front is adorned with eight 814400A-70 (70nS 1Mbit x 4 FPM) chips dated week 35 of 1993, made in Malaysia. The rear is adorned with four 81C1000A-70 (70nS 1Mbit x 1 FPM) dated Week 29 of 1993, made in Singapore. This gives the module a total capacity of 4 Megabytes with parity.
Note that the presence detect jumper solder pads are visible but not populated.
Other manufacturers of memory existed too – this one was a Seiwa Japan made module, 23216 Rev A, made with 16 Siemens HYB514400BJ-50 (50nS 1Mbit x 4 FPM), made in Germany. Half are date coded Week 34 of 1996, the other half Week 28 of 1996. This board even has the presence detect jumpers fitted, and bypass capacitors for each chip. This makes the module an 8 Megabyte module without parity.
This module is a paretty old module, it’s actually quite rare to find a yellow coloured module (in my memory, at least). This was a Kingston KTM-8000/M70 module marked with KTA-CENT/8 which implies it was a compatible 8Mb memory module for an Apple Centris server. Its PCB is etched with a copyright date of 1991.
It is assembled from 16 Hitachi Japan HM514400AS7 (70nS 1Mbit x 4 FPM) chips dated Week 38 of 1992, all arranged horizontally rather than vertically. There are also unpopulated positions for (likely) parity chips, which take on a form factor I haven’t ever seen!
The presence detect jumpers seem to be fitted, with one bypass cap per two memory chips.
This one is a module dated 29th November 1995, sold November 1996, made from 16 Hynix Korea HY514400A J-70S (70nS 1Mbit x 4 FPM) memory chips dated Week 43 and 44 of 1995. It does not have its presence detect jumpers populated, nor does it have bypass capacitors for each chip, as it seems to be a cost saving measure. The module itself is hence a 8 Megabyte module without parity.
Many of the SIMM memory makers have been featured in this collection, but I’m sure I have seen OKI, Goldstar and Mistubishi as well, but don’t have them in my possession anymore. It’s been a long time since I’ve last seen them, but there may still be one or two lingering in a dark corner.
Of note was that SIMMs came in gold fingered and “tin” finger configurations, with advice to avoid mixing gold socket pins with tin fingered modules and vice versa due to oxidation concerns. In my experience, I didn’t experience much issue either way, but I didn’t see the intelligence in advice such as “use an eraser to clean the contacts of the SIMMs” – that might develop enough static charge to kill them!
There’s a tour of the collection of spare SIMMs that I presently have, and it covers a good range of chip manufacturers, but not so much in terms of different SIMM capacities. It’s crazy to think that in the early to mid 1990s, the modules numbered in the single digit number of megabytes. In just twenty years, we have modules numbering in the quadruple digits of megabytes. It definitely shows you the level of progress, as well as how “simple” these memory modules were – presence detect jumpers instead of EEPROMs, contacts with wide spacing, redundant on both sides. Even the IC packages themselves are very thick compared to the wafer thin ones of today (not pictured here, but maybe later, if I ever bother with an “SDRAM DIMM” collection). What a world we live in!