Replace 70ns Memory IC with 12ns?

[Phil Davies]

I have been given a piece of diagnostic equipment which suffered pcb and component damage, the previous owner attempted a repair but it still didnt work and gave it to me.

I found a few tracks which were still broken and a couple of poor solder joints but have a question on the SRAM IC's, the original items were 55TC1001 128 x 8 at 70ns access but he has replaced them with AS7C1024B 128 x 8 12ns devices.


Is this ok or will there be any problems?

Thanks

 

[FvM]

The question can't be easily answered. Presuming pin compatibility, the fast memory should basically work. But the circuit might generate non-ideal control signals with gliches or ringing edges. It can happen that the artefacts are ignored by the 70 ns chip but cause failure of the 12 ns device. I observed similar effects when changing from 120 ns to 70 ns chips in a pretty old design.

 

[Phil Davies]

Likewise this piece of kit is quite old now, circa 2000, the devices are pin compatible so i will stick with the 12ns devices for now to see how it goes, i also have an identical working unit which i can you use for reference.

thanks

 

[FvM]

That would be also my suggestion. If it doesn't work, inspect the signals, a small capacitor in the right place might be sufficient to slow it down.