Eprom OS Issues

[+Larry]

I have an 800XL that has recently had stability issues with some (but not all) PBI devices. I've seen this before, but not for a long time with my XL's and XE's. This 800XL has a 256 Newell upgrade, a 4-in-1 OS, and Super Video 2.1. It is actually the very first 800XL that I ever purchased at Zayre (if you remember them) for about $50.

 

The PBI instability issues appear to have been cured by the Puff Stable XL/XE fix #2. Since #2 deals with the eprom OS, it got me thinking -- what is the difference and why? I disconnected the "fixes" and took out the 4-in-1 OS (27C512 -- 90 ns) and replaced it with a single stock rom OS. The instability vanished. I put in a single 27128 (200 ns.) OS and still no instability. So what gives?

 

Is it the particular eprom? The switches? Are the "pull-up" resistors used with the 4-in-1 the real culprits? Could be all of the above? Let's get to the resistors... This is an old issue -- what is the "right value?"

Are there trade-off's implicit in this choice? If we use large value resistors, is it more likely that we'll have timing issues caused from the eprom?

 

I'm thrilled that my problem is fixed, but I'd sure like to have a better understanding of "why" so as to help avoid it in the future.

 

-Larry

[+bob1200xl]

Do we actually know what the 'instabilities' are? I tried a 300ns EPROM in a 1200XL the other day and it would not run. The specs for access time give us the worst case timing, not the average time. So, we can plug in a whole lot of things that work, even though the guaranteed timing is no good. A 300ns EPROM should work. Maybe some do. That doesn't mean that the chips are bad - it means that we don't have enough information to get it right.

 

A 90ns EPROM may be too fast. It may react to garbage on the address lines or tri-state the outputs too quickly. We don't know, really.

 

As far as the resistors go, they are static, aren't they? The value (within limits) will only matter if you are switching them at clock speeds. Some switches will kill the system if you tap them sharply.

 

Bob

 

 

 

I have an 800XL that has recently had stability issues with some (but not all) PBI devices. I've seen this before, but not for a long time with my XL's and XE's. This 800XL has a 256 Newell upgrade, a 4-in-1 OS, and Super Video 2.1. It is actually the very first 800XL that I ever purchased at Zayre (if you remember them) for about $50.

 

The PBI instability issues appear to have been cured by the Puff Stable XL/XE fix #2. Since #2 deals with the eprom OS, it got me thinking -- what is the difference and why? I disconnected the "fixes" and took out the 4-in-1 OS (27C512 -- 90 ns) and replaced it with a single stock rom OS. The instability vanished. I put in a single 27128 (200 ns.) OS and still no instability. So what gives?

 

Is it the particular eprom? The switches? Are the "pull-up" resistors used with the 4-in-1 the real culprits? Could be all of the above? Let's get to the resistors... This is an old issue -- what is the "right value?"

Are there trade-off's implicit in this choice? If we use large value resistors, is it more likely that we'll have timing issues caused from the eprom?

 

I'm thrilled that my problem is fixed, but I'd sure like to have a better understanding of "why" so as to help avoid it in the future.

 

-Larry