May 1999 Slot-1 BX Motherboard Roundup

ABIT’s introduction of the original BX6, one of the first BX boards to hit the market, boasted a handful of new and "revolutionary" features that went unmatched by any competing motherboard manufacturers. Among the features was the inclusion of a 6-chip external DRAM buffer, which was designed to help improve stability when using all 4 DIMM slots. ABIT never officially stated that the buffer would in fact increase stability, nor did they ever confirm that the DRAM buffer was necessary, they just included the buffer and dubbed it a "feature" on the BX6.

Now, a little under a year after the BX6’s introduction, the truth happens to be that the 6-chip external DRAM buffer does absolutely nothing for the stability of the board when using all 4 DIMM slots. The idea behind the buffer is simple, when the power drawn by the memory banks increases, the power fed to them as well as the clarity of the electrical signals feeding them must remain as clear as the signal present when only one DIMM is installed. As you increase the amount of resistance an electrical signal must fight in order to reach the other end of a circuit (in this case traveling to/from the 443BX North Bridge controller from/to the memory banks) there are issues of capacitance that must be taken into account. While some manufacturers have insisted that a DRAM buffer isn’t necessary to help maintain signal strength and clarity, by saying that a reduction in the trace length between the DRAM banks and the 443BX controller is all that is necessary (the distance between the main chip of the BX chipset and the memory banks) there are others, such as ABIT, that claim the 6-chip DRAM buffer is a necessity to prevent any capacitance related issues with the motherboard’s stability.

There is some truth to this, basically as a motherboard manufacturer you want to have the memory banks as close to the memory controller (443BX chip in this case) as physically possible to keep the length of individual traces between the two down to a minimum, however the addition of the DRAM buffer doesn’t necessarily remove the need for that.

In AnandTech’s own internal testing, the ABIT BX6 and the newer BX6 Revision 2.0 performed considerably worse (in terms of stability) with 4 – 128MB DIMMs of the same type than other motherboards that lacked the DRAM buffer. Although your mileage may vary depending on the type of DIMMs used, something to keep in mind when purchasing a BX motherboard is that maxing out all of your memory banks isn’t encouraged unless you happen to have a high-end server board or a board with a proven stability track record. DRAM buffer or not, ABIT’s BX6 isn’t a board you want running in a server where every crash counts against you.

With the growing need for larger memory configurations, many memory manufacturers are turning to slightly unorthodox yet popular ways of releasing 256MB and larger DIMMs. The problem memory manufacturers have is this, on your standard 64MB SDRAM DIMM you have a total of 8 chips (non-ECC). Basic math will tell you that 8 chips x 8MB per chip gives you a total of 64MB, the total size of the memory module. Since these 8 chips only occupy one side of the double sided module, the second side can be populated with another 8 chips to make a total of 128MB (8 chips x 8MB per chip x 2 sides). The problem comes in when you want something larger than 128MB, there are no 3^rd and 4^th sides to include 8 more chips on for a 256MB module.

So what do you do? The skyscraper method, build upwards. Hitting 256MB requires that you essentially stack another 128MB module upon an existing 128MB module, mainly because 16MB and 32MB SDRAM chips are not plentiful (nor cheap) enough for manufacturers to start releasing double sided 256MB SDRAM DIMMs made up of 16 chips of 16MB a piece. In turn, this causes a severe problem with motherboards, since the profit margin on a single motherboard is quite low, every corner must be cut and every dollar must be saved during the manufacturing process. When you combine a 256MB SDRAM DIMM (essentially made up of 2 - 128MB DIMMs) with a motherboard that wasn’t tested with that particular module, you get a major problem. In most cases, your system will be extremely unstable or will simply fail to boot, giving you a memory error beep code.

The problem is even worse with 512MB DIMMs. The solution? Unfortunately, the solution is to either buy smaller memory DIMM modules or a motherboard that can handle the particular DIMMs you are looking to use. Companies like Tyan, that manufacture motherboards intended for use in high-end configurations, have put together recommended SDRAM lists based on modules that have been known to work properly with their motherboards. Your best bet in that case would be to consult your motherboard manufacturer before popping in a few hundred bucks worth of SDRAM in your system that may or may not work. From personal experience, one of the most frustrating things is when your $3000 512MB SDRAM module won’t work in your server motherboard, and there isn’t a 256MB memory vendor around on a Sunday night. The things that go on behind the scenes at AnandTech ;)