Queery In Regards To Ram Cas Latency

Hi Guys, I think I am confused here, my GA-K8VT890-9 Motherboard has some issues with CL.
I had been of the opinion that CL was backward compatible just like the speed. I can put PC 2100, PC 2700 or PC 3200 into this board even mix them up and it will post and work as long as they are CL2.5 (don't know about 2 as I don't have any) but as soon as I place one stick of CL3 into the board I will not get a post.
And there is no option in the BIOS to change that, and no mention in the manual about it neither.
I could understand that if the BIOS would be set for CL3 that it might not work with the quicker 2 or 2.5 but not the other way round.
Any ideas on that.
hawk22

 

Yes, you've got it a little mixed up. Lets draw an analogy and say Sprinter #2 can run 100 Meters in 10 seconds while it takes Sprinter #3 12 seconds to cover the same distance. Now, no matter what Sprinter #3 does, his fastest time is never going to improve - thats the absolute best he can do. On the other hand, Sprinter #2 can run a 10 second 100M or he can coast along and run a 12 second 100M.

CAS Latency (Column Address Strobe) is only one part of the whole memory equation and collectively it can be a very complex subject. Here's a link to TechARP (the free version), which used to be Adrian's Rojak Pot. There is no better guide to the BIOS including any and all memory questions unless you want to subscribe to the considerably more detailed paid version of this site. Head over there and spend some time reading.

 

Thanks Rockster, great link that is.
Now please be patient with me here, while I garble on a little bit more.
My line of thought was actually running along those lines as you describe by runner #2 and #3, thinking that if 2 sticks of ram are inserted #2 is PC 3200 and #3 is PC 2100 #2 will slow down to run alongside #3. Therefore CL2.5 would slow down so that CL3 can keep up with it. That was my simplistic view.
I did a scan at the Crucial site and it tells me that the board can run CL2.5 or 3 but even if I only insert 1 stick of CL3 on its own I will get no post.
Could the Dual Channel Memory have something to do with that.
hawk22

 

There's your key, now what might that tell us? Fundamentally, it could mean that your motherboard and processor don't like the slower CAS3 memory or the module's actual bus speed of 266MHz (133x2). If I were to guess however, I'd say you have a cheap high density stick of PC2100. Read on ...........

Now, just as Christer suggested re: single vs dual channel memory read and writes, there may be a tweakable setting in your system's BIOS that provides for adjustment. Then the question becomes, how am I supposed to change a setting in the BIOS if I can't even boot? The only answer is to first clear CMOS and then try the CAS3 module by itself. That may or may not work but its an option that must be pursued to determine whether or not the module is compatable. Some newer boards may not require this step but that little CMOS jumper will become very familiar to those who insist on extracting every tiny bit of performance they can get out of a system.

Unfortunately, these aren't the only determining factors at play here. I'm going to assume both of your sticks are unbuffered and are not ECC or registered memory but one also has to look at whether its high or low density. To keep things simple, high density memory is more often problematic than not and its usually cheaper than low density memory. You can Google this for more detail but a good indicator is reading specs like 16x32 compared to 8x64 with 8x64 being the higher density in this example.

I've never been an advocate of mixing and matching memory but this becomes even more critical in a dual channel board. One should buy matched pairs and matched pairs only. Sure, it's possible to get something to run on odd's and end's, but chances are good that even if it runs, a mix & match machine is going to throw a lot of errors.

Your best source of understanding is still going to be that link I provided earlier but it really requires a paid subscription to get the kind of detailed explanation you might be looking for. Then again, it borders on information overload and confusion because of the detail provided.

The last thing that should probably be mentioned is the power requirement. 2.5 Volts is common for the DDR modules you have referenced but some will require 2.6V or 2.7V to run according to optimum specifications. Again, this isn't going to affect the common Joe, but when one gets into tweaking and fine tuning for performance it can be the difference between night and day. Some memory that is advertised as 2-2-2-5 will not run at that speed until the voltage is set properly (usually manually) even though the module itself has been correctly identified and recognized by the motherboard's BIOS. Its not uncommon to push this even higher when overclocking.

Hawk - this can be a very confusing subject and it is one that few people truly understand. The best takeaway I can provide can be summarized with one simple sentence. Not all memory is created equal.