RAM expansion cross-talk issues (split from: Bitshifter's Z-code interpreter)

[eslapion]

The only timing related device I am aware of in the VIC was the 555, so I thought I'd ask.

The 555 is not related to timing. It just generates a pulse which, in the case of the VIC-20, is used to generate the reset signal when the power gets turned on.

The only timing related device in the VIC-20, is the crystal and related logic circuits which are connected to the 6560/6561.

[Kakemoms]

For the love of god!! Where did you guys get such nonsense?

The 555 is only responsible for generating the startup reset pulse and the 74LS138 divides the VIC-20 memory areas in various sections. Neither of them has anything to do with the VIC-20's timing!

Anyways, I couldn't find Kakemoms original post to know exactly what this is about...

The clock signal can be found on pin 38 of the 6560/6561.

You should read the original post. Anyway, all logic components have delays which affect the timing of the signals. As for the 74LS138N it is the source of the BLK signals that is used as a chip select for SRAM chips in expansions. If the CS signal isn't active at the correct moment, no response will be heard from the SRAM. In the case the CS signal is active, the SRAM will respond. Now, if you happen to get a CS a little late, the SRAM may respon or it may not respond. That generates some confusion on the databus, and you may not see the correct value from a PEEK.

I have no idea on how 74LS138N ages, but it can certainly make less sense if it drifts out of specs.

[eslapion]

You should read the original post.

Since the thread was split, I could find it.

Anyway, all logic components have delays which affect the timing of the signals. As for the 74LS138N it is the source of the BLK signals that is used as a chip select for SRAM chips in expansions. If the CS signal isn't active at the correct moment, no response will be heard from the SRAM. In the case the CS signal is active, the SRAM will respond. Now, if you happen to get a CS a little late, the SRAM may respon or it may not respond. That generates some confusion on the databus, and you may not see the correct value from a PEEK.

I have no idea on how 74LS138N ages, but it can certainly make less sense if it drifts out of specs.

Combinatorial logic 74LS series chips usually respond in 20ns or less. I have some from the 1970s and I have some that were purchased less than 2 years ago and they have almost exactly the the same response speed.

The speed of the VIC-20 (the duration of it's access cycles) makes it compatible with antiquated ordinary diode-resistor logic which can take up to 200ns to respond. There is a DRL gate on the Behr-Bonz and it does the job just fine.

Drift out of spec?

Capacitors can drift out of spec when they have a dielectric which is based on organic compounds that can degrade over time. Monolithic semiconductors get out of spec when they fail!

The 74LS138 of the VIC-20 is actually more than 10 times faster than it really needs to be, unless it's ... dead.

[norm8332]

Monolithic semiconductors get out of spec when they fail!

I just want to note that I did have a 244 line driver on a C128's RGB output that was causing a distorted output when the screen was half filled and beyond, but with only a few lines it was fine. The chip tested fine when each buffer was tested individually in a tester, but when heavily loaded it failed. I'd call that a partial failure. A reminder to me that digital is within analog constraints.

I had cross talk issues recently when working on my "poor man's 35K upgrade" and it turned out that I was using the wrong line for /WE. I was using CR/W instead of VR/W, but you set me straight in a old post I found on here.

[groepaz]

you cant rely on those testers, not seldomly oldish chips work fine in a tester that only puts very slow signals on a chip, but on a mhz or two it fails

[Kakemoms]

Combinatorial logic 74LS series chips usually respond in 20ns or less. I have some from the 1970s and I have some that were purchased less than 2 years ago and they have almost exactly the the same response speed.

The speed of the VIC-20 (the duration of it's access cycles) makes it compatible with antiquated ordinary diode-resistor logic which can take up to 200ns to respond. There is a DRL gate on the Behr-Bonz and it does the job just fine.

Drift out of spec?

Capacitors can drift out of spec when they have a dielectric which is based on organic compounds that can degrade over time. Monolithic semiconductors get out of spec when they fail!

The 74LS138 of the VIC-20 is actually more than 10 times faster than it really needs to be, unless it's ... dead.

LOL. That is utter non-sense, and I know since I have worked with lifetime testing of semiconductor devices for many years.

There is no such thing as devices with unlimited lifetime, since any semiconductor will degrade while it is being used. There are a number of processes at play (electron drift, oxidation, mobility degradation), but main thing is that at some point it they will all FAIL. FAIL is defined as being out-of-spec, or for the discussion at hand, you can define it to being too slow to reliably work with the other ICs.

Nothing lasts forever and certainly, 74LS chips manufactured in 1983 were not always manufactured with lifetime measurements as part of the manufacturing process. And that was especially true for Vic-20 production since it basically vacuum cleaned the market and gave a component shortage. Inside a Vic-20 you never know which manufacturer to find, and some may not even be from a certified production run. As you said, they were 10 times faster than needed to, so a little "slip" with timing would not make any problems.

So no, semiconductors age and fail. They are not prone to aging.

[eslapion]

I just want to note that I did have a 244 line driver on a C128's RGB output that was causing a distorted output when the screen was half filled and beyond, but with only a few lines it was fine. The chip tested fine when each buffer was tested individually in a tester, but when heavily loaded it failed. I'd call that a partial failure. A reminder to me that digital is within analog constraints.

I suspect the problem is actually with impedance load limits. Every 74LS IC can only drive a certain amount of current. If this limit isn't exceeded on a single output but gets exceeded when multiple outputs are driving then there will be a collapse of the output current.

There is no such thing as devices with unlimited lifetime, since any semiconductor will degrade while it is being used. There are a number of processes at play (electron drift, oxidation, mobility degradation), but main thing is that at some point it they will all FAIL. FAIL is defined as being out-of-spec, or for the discussion at hand, you can define it to being too slow to reliably work with the other ICs.

It seems you've seen things I never have.

In the specific case of 74LS ICs I have never seen one of them 'get out of spec' as in 'get slower'. IMHO, they work or they don't, just like an incandescent light bulb they age and stop operating completely. As your previous post implied, I have never seen one become slower as a result of aging.