|I too have found that some ram manufacturers are listing modules with high-density chips for a system that WILL NOT ACCEPT high-density chips, but I have found that ONLY for mboards with older main chipsets.|
E.g. For mboards with the the Via MVP3 and MVP4 main chipset series, Kingston shows in their *.pdf for the ram module specs for non brand name system modules as having 16 chips for a 256mb SDram module, yet new ram with the exact same module part number has 8 chips, and isn't recognized properly.
Similar applies to the earlier main chipsets that could use 512mb SDram modules - the module must have 16 chips, not 8.
However, I don't think the problem is as widespread as you are saying.
The ram can also be incompatible for other reasons.
- It may not be compatible with being used along with other existing ram modules because the other module(s) is(are) not 100% compatible. In that case, each ram module may work and test fine when it is installed by itself, but the combo of ram modules doesn't work properly.
- for newer ram, there may be other reasons a combo of modules doesn't work properly, or the bios may set ram settings wrong by default.
Some ram manufacturer's modules do not strictly adhere to the JEDEC standards that most mboards bioses use to determine ram settings.
In that case, the ram settings in the bios Setup that the bios has automatically chosen may not be correct.
Check the ram settings in your bios - the ram voltage, and the ram timing numbers - those should be the same as for the specs for the modules themselves. Often the ram voltage and timing numbers are printed on the label on the modules.
If the voltage setting or timings settings in the bios are different from the specs for the ram, change them in the bios. The timing numbers must be as close as you can get to the same, or slower timings (higher numbers = slower) - you won't notice the difference the slower settings make.
If you have a mix of different modules
- don't mix ram that different voltages are specified for - the bios will force the ram to use the lowest voltage, if "by spd" or similar is used (default settings) - ram that a higher voltage is specified for is more likely to not work properly in that situation.
- the bios settings must be those for the slowest timing settings of all the modules, or slower (higher numbers = slower).
You MAY be able to custom set the ram voltage to the higher ram voltage in the bios if you do NOT have the bios set to detect the ram "by SPD" or similar, however, you must NOT exceed the max voltage range for the modules that require a lower voltage, and that can be hard to determine, unless you can find detailed specs - e.g. if the ram is Kingston ram that doesn't have a brand name system specific part number, that info is easily found.
E.g. most DDR2 ram modules rated slower than 800mhz (slower than PC2-6400) use the standard JEDEC voltage, but 800mhz and faster (PC2-6400 and higher) modules can often use a non-standard voltage.
NOTE that we have seen cases where the ram can pass ram diagnostics tests when the ram timing numbers the bios is set to are WRONG (too low) - it's ONLY when you try to use an operating system in that situation that you have problems.
- another thing we have seen on this site is some "also ran" module makers such as G-Skil and PNY rate the specs for a module for when only one module is installed in a mboard to make the ram specs appear better, and/or they may not have actually tested their ram modules in all the mboards they list ram for. In that case, one module usually works fine, but more than one of the same modules may not - you may need to tweak the ram timing settings so that one or more of the timing numbers is/are HIGHER (slower) than stated for the module in the bios, or increase the ram voltage a small bit, in order to get more than one of their modules to work properly.