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Ikrananka

Dual BIOS - Suitable EPROM Types?

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I'm looking to mod one of my ColecoVisions with a dual BIOS. This obviously needs a 27128 type EPROM and many variants of this chip will be suitable. But, are there any specific Variants that should be avoided. For example does it matter if it a non-CMOS (27128) or a CMOS (27C128) type is used? What about timing, is there a minimum speed that is recommended, or are all available speeds suitable for the CV?

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I can't say for sure what works or won't, but here is what I know:

 

1) Most of the Coleco is LS-level technology, except for the processor with is CMOS. LS level logic defines up to 0.8V (absolute max) for lows and 2V+ for highs, with anything in-between being a crap shoot. Output highs are a minimum of 2.4V, and lows are a maximum of 0.5V.

 

2) CMOS levels are defined as fractions of VDD. So for 5V, low is up to a maximum of 5V/3, or 1.67V, while high is 2/3*5V or higher (3.33V+). The Z80 is CMOS, but actually adheres to the TTL levels. This is compatible, but not what you might otherwise expect. For instance, LS may have trouble driving a CMOS input due to its defined high level requirements. However, the Microchip 27C128 also claims to use TTL levels, so it's usable. Whatever chip you use, check the data sheet to make sure its levels are compatible with 1).

 

3) One clock cycle at 3.58MHz is about 270ns. In addition, the CV adds a wait state for M1, which seems to be the fastest memory access period. So the minimum allowed access time appears to be 2 cycles, or around 540ns. So pretty much any ROM will work speed-wise. All the ones I see today are around 150ns at worst. Actually, come to think of it, I have a vague recollection that the CV technical manual tells you what the slowest ROM you can use is.

 

So you'll probably be okay with whatever.

Edited by ChildOfCv

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So you'll probably be okay with whatever.

 

Sorry - I missed your reply. Thank you so much for the detailed answer. Awesome, I'll now go place an order :)

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