View Full Version : 74LS chip speed, s100 hard disk controller

February 25th, 2018, 11:50 AM
I'm curious about my Xebec S100 SASI host adapter. Most of it is 74LS TTL chips and it runs fine with a 6Mhz Z80 processor card in the bus. When I go to 8Mhz, then things break down, it will access the hard disk some, but eventually hang. Could it be possible to swap the 74LS chips out with faster 74AS chips and gain the ability to operate at 8Mhz? I don't know if this line of thinking has merit, anyone here have insight?


February 25th, 2018, 12:35 PM
I think it is unlikely that you can succeed by blindly swapping 74LS parts for 74AS parts. The AS parts will take more power and the faster [rise and fall time of the] edges may cause serious noise problems.

It might help if you really understood what is making the system hang. For example, does the faster CPU actually overrun the SASI bus and confuse the disk controller? Faster ICs on the host adapter wouldn't help in that case.

February 25th, 2018, 12:51 PM
What Uniballer said. Slower bus speeds can mask "race" conditions in the logic--and using faster logic could, oddly enough, exacerbate them.

LS logic should be plenty fast for an 8 MHz bus; however, a lot depends on what kind of load the logic sees.

February 25th, 2018, 01:40 PM
Potato Semiconductor makes 74 series logic that runs into the GHz range. Throw some potato chips at the problem and you should be fine.

February 25th, 2018, 01:50 PM
Thank you for ever increasing my understanding, love this forum!

February 26th, 2018, 11:10 AM
Does your System utilize dynamic ram board/s?

If so, I would think that timing problems with dynamic ram memory, and dynamic ram memory boards would cause a speed problem before the hard disk host adapter.

Early S-100 boards did not adhere to standards (since their weren't any way back then), and different schemes were used to trigger, and steal the time from the System for dynamic ram refresh that was not compatible with boards made by different manufacturers.

Are you sure that your processor board and other boards will support 8mhz?

February 26th, 2018, 12:08 PM
I'll add that you can replace 74LS chips with 74ALS or 74AS without too much of a ding on the power budget, rather than the older 74S or 74F parts. But it probably won't solve your problem.

February 26th, 2018, 06:12 PM
Are you sure that your processor board and other boards will support 8mhz?

As configured for testing, the only boards in it are a Teletek Systemaster SBC and the SASI Host adapter, so there's no interaction with other boards on the S100 bus. I have a few experiments to try and I also now have access to a SCSI bus analyzer which could be useful, I just need to study up on how to interpret the logs it captures.