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[Install] Having two sets of memory with different CAS

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June 8, 2011 6:44:50 AM

Hello :) 

So I recently bought a used computer. Dell optiplex 745 and upgraded the ram (2x512mb cas 5) to something a little faster and more spacious (2x1gb cas 6) .

I always thought that having two different sets of ram would reduce speed, so I took out the old ram. (2x512mb cas 5)

As I was reading the manual, to see where to properly install for dual channel. It made me wonder if the computer could run cas 6 and cas 5 at the same time.

http://support.dell.com/support/edocs/systems/op745/en/...

What do you guys think? Leave the old ram in or just sell it on ebay?
a b } Memory
June 8, 2011 12:49:54 PM

I think that you are wise to sell the old off and, if you want to supplement your RAM buy a new pair the new with identical timing. Using RAM with different CAS is an invitation for trouble, and I don't think that it can even be done. How would the mobo differentiate between the two speeds? Most tech articles I have read even say that you should not even use diffenent RAM sizes (i.e. mixing 512 & 1024 is a no-no).

Don't do it lest you fry your rig.
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June 8, 2011 3:26:32 PM

the CAS dont matter one bit really. just go in the bios and set it to depending on what type of memory. what is the speed and cas of your 2 memory modules
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June 8, 2011 9:22:25 PM

It is generally frowned upon to run 2 different kinds or latencies of ram. Yes on some mobos you can manually set the speed and latencies to be the same. But even doing this can lead to trouble.

However you will not fry your rig or cause any physical or permanent damage by running different kinds of ram. You will run the risk of your ram having poor performance or causing lock ups and BSOD.

Overall you will be better off just sticking with new ram that you just bought and leaving it at that.
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Best solution

June 8, 2011 9:39:38 PM

WINTERLORD said:
the CAS dont matter one bit really. just go in the bios and set it to depending on what type of memory. what is the speed and cas of your 2 memory modules


I would recommend against this approach. Although you will not damage your memory or computer by running memory with different stock timings, you may find that your system is not stable.

Some memory sticks can do 9-9-9-24 at 1.5v, but some need 1.65v to do that same timing while at the same time guaranteeing stability. Since you can't set memory voltage individually, you risk overvolting half the memory to accomodate the voltage requirements of the other set.

I would recommend selling the old memory -- you'll probably pick up a couple of bucks on eBay -- and buying a set with timings matching your new memory. Best way to get a stable system.

Good luck!
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June 8, 2011 10:39:36 PM

it all depends, is the memory ddr2 or ddr3? that makes a huge difference but if you set the timings manuly you should have no problems
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June 9, 2011 4:11:57 AM

fullofzen said:
I would recommend against this approach. Although you will not damage your memory or computer by running memory with different stock timings, you may find that your system is not stable.

Some memory sticks can do 9-9-9-24 at 1.5v, but some need 1.65v to do that same timing while at the same time guaranteeing stability. Since you can't set memory voltage individually, you risk overvolting half the memory to accomodate the voltage requirements of the other set.

I would recommend selling the old memory -- you'll probably pick up a couple of bucks on eBay -- and buying a set with timings matching your new memory. Best way to get a stable system.

Good luck!


One Question, is it necessary to match timing and latency both for example one stick 8-8-8-24 CAS 8 and other is 8-8-8-24 CAS 9 Can these two be used simultaneously or is it necessary to match timing and latencies?

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June 9, 2011 4:21:45 AM

mrhercules said:
One Question, is it necessary to match timing and latency both for example one stick 8-8-8-24 CAS 8 and other is 8-8-8-24 CAS 9 Can these two be used simultaneously or is it necessary to match timing and latencies?


The motherboard's bios likely doesn't allow you to set the latency of one set and the latency of the second set separately. At least, that's the way it works with my P8P67.
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June 9, 2011 4:24:25 AM

fullofzen said:
The motherboard's bios likely doesn't allow you to set the latency of one set and the latency of the second set separately. At least, that's the way it works with my P8P67.




Right, thanks.
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June 10, 2011 3:42:51 PM

WINTERLORD said:
the CAS dont matter one bit really. just go in the bios and set it to depending on what type of memory. what is the speed and cas of your 2 memory modules


Thank you for the response.

The upgraded ram is 1GB DDR2 800 (PC2 6400) with a Cas Latency of 6

The old ram is 512MB DDR2 533 (PC2 4200) with a Cas Latency of 5

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June 10, 2011 3:43:28 PM

Best answer selected by johnkerry.
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