Relationship between cas latencies for DDR1, DDR2, DDR3, etc

[Dubbs]

I'm just curious what the relationship between the CLs of DDR, DDR2, etc is. I know back in the day a CL of 2 was good for DDR1, and now a CL of 4 is pretty good for DDR2.

By that token, is a CL of 6 or 8 good for DDR3? Seems like the relationship is either x2 previous CL or 2^(DDR number). Or is there no relationship and CL of DDR1 completely differs from DDR2 from DDR3?

Just curious, no particular reason why

[VeoDigital]

The Ultimate Ram Guide tells it all. Here is the section on cas latency:

Timings vs. Speed (throughput):

It is important to note that all timings are relative to clock cycle. What this means is, the values stated for all the timings below are only valid for the speed that they are currently running. It is helpful to know how to calculate latency in nanoseconds in order to have a truly accurate measure of latency. For example:

If you have a DDR module that is rated for 2-2-2-6 at DDR-400, the actual CAS latency in nanoseconds is calculated thusly:

(1)/(Bus Speed) = Time period for one cycle

So:

(CAS Latency in cycles)*(1)/(Bus Speed)= CAS Latency as a period of time.

Simplified:

(CAS Latency in cycles)/(Bus Speed) = CAS Latency in time.

To take our above example:

(2)/(200,000,000) = 10ns.

It is more correct to say, that in this case, we have a CAS Latency of 10ns.

What we can do with this value is, we can compare it to say, CAS 2.5 at DDR-500:

(2.5)/(250,000,000) = 10ns.

Contrary to popular myth, CAS 2 @ DDR-400 is identical to CAS 2.5 @ DDR-500 in terms of latency. A quick rule of them for judging latencies is as follows;

CAS 2 DDR-400 = 10ns
CAS 2.5 DDR-500 = 10ns
CAS 4 DDR2-800 = 10ns
CAS 5 DDR2-1000 = 10ns
CAS 6 DDR2-1200 = 10ns
CAS 7 DDR3-1400 = 10ns
CAS 8 DDR3-1600 = 10ns

All of the above are identical in terms of latency. The same method can be used to compare other latencies as well.

[Dubbs]

Ahh got it. I clicked on the sticky for Guide to Cas Latencies and didn't see an answer. Dumb me didn't look in the other stickes..

[VeoDigital]

Cas latency is really just relative to the bus speed, 2 is no better than 4, depending on the bus speed.