Emu XL-7 & MP-7 User's Group

RAM is volatile memory [contents lost when power fades] unless you have a battery back up circuit. It is used for working values in software/firmware. On the P2K series modules the RAM was provided in the form of a 72-pin SIMM DRAM module which is at the opposite end of the board to the sound SIMM slots. The P2K series supports modules of 4, 8 or 16MB. In the non-turbo versions that was a 4MB SIMM which is fine for most things but since the P2K OS loads all of the parameter memory into the DRAM at startup (to prevent conflicts with accessing that content while the sound memory is being used) there are some combinations of sound SIMMs that can't be used at the same time. The turbo versions usually had an 8MB SIMM which is fine for any combination of sound SIMMs. I have come across some turbo boards with only 4MB of DRAM and they needed upgrading to 8MB to fully support 4 sound SIMMs. There is not any need to have a 16MB DRAM SIMM (unless new sound SIMM were produced making more use of the parameter memory), but the OS does support SIMMs that size.

On the command stations there is no module for the RAM memory. Instead there is an SDRAM part soldered onto the motherboard. SDRAM varies a little from DRAM in that the 'S' stands for "synchronous" meaning that the contents are output in sync with a clock signal. The main reason for this is the difference in the microprocessor used for the two different boards with the newer one (on command stations) supporting the faster SDRAM ad the older one not. A single part is cheaper than the module. The SDRAM part is 64Mbit (that is 8MB) and since it is soldered to the motherboard (TSOP II - 54 package I think) can not be upgraded. Since DRAM itself has gone out of favour SDRAM is often called DRAM though the interfaces to the two are not compatible [without additional logic to work around the clock signal].

It is WRONG however to call FLASH memory 'RAM' since it is not randomly writeable as true RAM is. It is more correct to call it 'FLASH ROM' (since it can only be written under special circumstances and is effectively read-only), but easier just to call it 'FLASH'.

As for SDRAM failure... It tends not to fail. If it does fail then there are two likely most likely causes (and radiation is not one of them, especially with the board shielded in a faraday cage - the metal case of the module).�

The most likely cause of failure is static damage. The parts are very sensitive to static damage and the amount of static electricity carried on a human body is enough to damage the chip. This generally takes the form of a damaged driver with the inability to drive the output level correctly. This is most manifest in burst mode transfers (using one address to access the memory contents of that address and then using the clock to 'pump' out consecutive locations in the next few cycles) when the output driver is not able to 'drive' or 'push' the output values fast enough. If the damage to the drive is small then it is possible for this to appear intermittently but it doesn't tend to worsen. The only fix for this sort of damage is to replace the chip.

The next most likely cause of SDRAM failure is a dry solder joint. Residual flux in solder can eventually cause the electrical connection between a pin and the pad its connected to to fail. Often this appears more frequently as the joint heats up (which can be quickly when the electrical connection is poor since it has increased resistance). It is often not possible to detect (by eye) a bad solder joint and reworking the solder (reheating the solder with a soldering iron to improve the connection) is the only way to find them since the problem is suddenly "fixed" after the rework. Because of the nature of the problem, dry joints tend to worsen over time (albeit slowly).

It would be worth noting that the problem of static and dry joints could apply to either side of the electrical connection (ie the SDRAM or the microprocessor). Additionally the microprocessor may have generated the error for some other reason including a transient noise spike in the system which might nor re-occur.

However if RAM is the problem - if the error appeared after some handling of the board (changing a sound SIMM or opening the case for some other reason) then I would favour static damage as a cause. If the board has not been handled (ie the module not opened) in a long time then dry joint would be higher on the list of causes. A single SDRAM chip is not terrible expensive ($5 or so from mouser) so if you are feeling adventurous and are handy with a soldering iron you could replace the chip your self (which would also rework the solder joints). However for most people, replacing the motherboard is a far easier option. And further, if the problem is very intermittent (as it seems) it may be worth ignoring until it becomes a practical problem.