CAS 2.5 = which timings???

[p0lar]

If the proposed RAM is PC2700, and it advertises CL2.5... can it run CAS 2-2-5-2 @ 133MHz speeds?

I'm currently using Mushkin PC3200 that runs (with ease) CAS 2-2-5-2 in the BIOS of my DP533-S.

If I switch to the PC2700 that is CL2.5, will I lose any performance if I never plan to overclock these particular processors?

Does it matter if it's dual channel RAM or not, as to what RAM I buy as long as it is in identical pairs?

Thanks for your advice/tips/info!

 

[pcman83]

cas 2.5 means its going to run something like 2.5-3-3-6 or something the cas latency will be 2.5

 

[p0lar]

So does that make my Mushkin CAS 2.0 ?

Why all the confusion? Why not have it label all the CAS timings on the RAM instead of this 2.0, 2.5, 3.0 business? RAM used to be just CAS2 or CAS3, now it's vague - misleading at best IMHO.

Also, since it's PC2700, that means its rated for 2.5 (whatever that is.. *barf*) @ PC2700 speeds or roughly 166MHz (if memory serves). So, at 133MHz speeds, will it do the 2-2-5-2 that I'm currently running on the Mushkin or is there no chance?

Why so much confusion? Or is it a plug-and-pray situation?

 

[Matthias99]

I don't see what's so confusing about it -- the latency is either 2.0, 2.5, or 3.0 clock cycles *at the rated speed*. Now, if you run it at a lower speed, you should be able to use tighter timings, and if you run it at a higher one, you'll probably have to use looser timings. You can calculate the required speeds pretty easily:

CL 2.5 at PC3200/200Mhz (5ns clock step) = 12.5ns latency, and CL2 is 10ns. At 220Mhz (4.5ns clock step), CL2.5 requires RAM with a latency of 11.25ns, and CL2 requires 9ns latency (CL3 "only" requires 13.5ns). So we can see that PC3200 CL2.5 memory should run at CL3 at 220FSB. PC3200 CL2 RAM might make CL2 at 220FSB, but would most likely have to be run at CL2.5 (which it should do easily).

Along the same lines, at PC2700/166Mhz (6ns clock), CL2.0 requires only 12ns RAM. So a PC3200 CL2.5 chip will usually be able to run CL2 at PC2700 (I hope there's a half-ns of headroom built in!).

RAM timings for individual chips are usually specified in terms of ns (so that they're independent of FSB speed), but since DDR RAM is usually sold at a specific speed grade (and memory controllers work this way), it makes more sense to sell them in terms of clock cycles at a specific frequency.

Edit: slight typo in my CL2.5 numbers.

 

[p0lar]

And I do understand how that works; however, in my BIOS, I don't have the option for a 2.5 CL, only 2,3 and higher (with one of them being a 6 -- I'm at work at the moment so I can't exactly tell).

From memory, I recall it being 2-2-5-2.

So, by my calculations based on your data (and calculations):

PC2700 @ CAS 2.5 (166MHz, 6ns clock) = 15ns (rated speed of the RAM)
PC2100 @ CAS 2.0 (133MHz, ?ns clock) => 15ns

Thus the clock cycle of PC2100 (133MHz FSB) should be greater than or equal to 7.5ns for me to consider using this memory. Am I correct? If it is 7 ns, then if I run it at 2.0 then it will run 14ns, which is lower than the 15ns it is rated at and I run the risk of instability. I think I have the grasp on the math behind it, I'm just not sure what clock the PC2100 runs at.

If:
PC3200 = 5ns clock
PC2700 = 6ns clock
PC2100 = 7.2ns clock?

They are Samsung chips, if it matters. /=

Thanks for the help with the calculations, Matthias, am I on course? If so, a 14ns should be feasible if it's quality RAM without too much concern for instability.

 

[Matthias99]

And I do understand how that works; however, in my BIOS, I don't have the option for a 2.5 CL, only 2,3 and higher (with one of them being a 6 -- I'm at work at the moment so I can't exactly tell).

The CAS Latency is only the first number -- THAT one is either 2.0, 2.5, or 3.0 cycles. The other latencies are less important (and more dependent on the exact chips and memory controller in use), and will go up to 6 or more in increments of a full clock cycle. It's possible an older motherboard might not support CL2.5, I guess... I'm unfamiliar with the DP533.

Required clock rate (in ns) = 1000 / FSB speed (in Mhz). I leave the derivation of this formula as an exercise to the reader.

So for PC2100 (133Mhz):

One clock step = 1000 / 133 ~= 7.5ns (7.52ish, actually)
CL2.0 = 1000 / 133 * 2 = 15ns
CL2.5 = 1000 / 133 * 2.5 = 18.75ns
CL3.0 = 1000 / 133 * 3.0 = 22.5ns

Your PC2700 CL2.5 RAM should be able to handle CL2 at PC2100 speeds, although it may be something more like 2.0-3-3-6 or 2.0-4-4-8 than 2.0-2-2-5. Just raise the timings and run it through memtest and prime95 until it comes out clean.

 

[p0lar]

Ah, I see now.

The first strobe is the most important one, though, right?

Ok, I think I'm going to go through with the purchase. It wouldn't ordinarly matter so much, but I'm not willing to move from 1GB of RAM to 2GB of RAM (using 1GB sticks and ditching my 2 x 512MB sticks) if I'm going to lose performance and not actually dip into the second GB (it's a rare occasion that I do, but it happens - hence the purchase considerations).

The DP533-S is manufactured by IWill and is a dual xeon board based on the E7505 MCH, FWIW. I'm unsure of its age, but it's probably ~18 months old.

Thanks again!

 

[JBT]

I'm willing to bet if u stick some ram in there with CAS 2.5 that option will become viewable in the bios.

 

[p0lar]

For some reason, that seems unlikely considering what is currently inserted that runs quite easily at CAS 2 @ 100MHz (currently have 2 x 2.4GHz Xeons w/400MHz FSB). If it would run at 2.0, the bios would have certainly let me run it at 2.5 since it will let me run it at 3.

While I can modify the multiplier in the bios (from 14 - 24) I cannot modify the FSB except by physical jumper on the board itself (and only at 100MHz or 133MHz). This isn't a positive indication of a board that is receptive to modifications.

The reason it matters is because I've got a pair of 2.8GHz @ 533MHz FSB procs going in here in two weeks.

I'll see if I can get a picture this evening of what's going on in the bios.

 

[p0lar]

Ok, I found info on the chip!


Samsung's Site

So, I'm thinking there's a good chance I'll be able to pull the same timings I use with the Mushkin by looking at that "Speed @ CL2" line in the table towards the bottom..

 

[Matthias99]

Ah, okay. I didn't realize it was a server board. I'm willing to bet it just doesn't have a very overclocker-friendly BIOS (having to set manual jumpers is a bad sign, too), and CL2.5 support may just not be there. It's just not built for performance tweaking.

Ok, I think I'm going to go through with the purchase. It wouldn't ordinarly matter so much, but I'm not willing to move from 1GB of RAM to 2GB of RAM (using 1GB sticks and ditching my 2 x 512MB sticks) if I'm going to lose performance and not actually dip into the second GB (it's a rare occasion that I do, but it happens - hence the purchase considerations).

Keep in mind that the difference between CL2 and CL2.5 memory is pretty small, whereas having to go to a swapfile is orders of magnitude slower. I don't know what kind of applications you're running on this box, though. Unless I was doing something like photo/video editing, rendering, CAD/CAM work, or a large DB project, I'd probably stick with 1GB at tight timings.

 

[PCTweaker5]

Just a question, but is 2-2-3-4 a good timing for RAM? Im just unsure of what the last number is all about, is it really good to keep it low? I tried 2-2-2-7 but system wont boot at that speed.

 

[PCTweaker5]

fvk j00 den

 

[p0lar]

fvk j00 den

Calm down, j00 l33t h4x0r j00...

Why post garbage like this to crap an otherwise informational thread? This is *almost* surprising from someone with > 500 posts.

 

[PCTweaker5]

lol I thought this thread was dead so I figured nobody would read it soon but I was just messing around I wasnt pissed. Lol

 

[0roo0roo]

http://www.tomshardware.com/motherboard/20040119/index.html