Interface 1 with 128K Spectrum

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daveZX4th
Posts: 16
Joined: Mon Dec 04, 2017 2:22 pm

Interface 1 with 128K Spectrum

Post by daveZX4th »

I have a 128K Toastrack Spectrum which I haven't used in a while (just needs new membrane). Back in the day it wasn't too reliable when Interface 1 (IF1) and microdrives were attached, whereas my 48K Spectrum+ worked flawlessly with IF1 and microdrives.

From what I recall the problem was that the Z80 could only drive about 5 LS TTL loads, which meant the 48K Spectrum had a couple spare for the expansion port IF1 ULA and ROM chips. But the 128K adds the AY and extra ULA chips inside which effectively leaves nothing for the expansion port. Hence anything added to 128K expansion connector strays into the no-mans-land of logic levels normally meant for noise immunity. So although it worked most of the time occasionally it would crash. I've done a bit of searching on web but can't find anything regarding this problem, I'm sure it wasn't just me that encountered it.

Although I have a working IF1 on 48K Spectrum I also have a spare IF1 ULA. I was considering building a replica IF1 using HCT buffers to reduce load on Z80 buses. I have the know-how to do it but I would be interested to hear if anyone knows if it has been done before.
cj7hawk
Posts: 8
Joined: Sat Oct 09, 2021 7:43 am

Re: Interface 1 with 128K Spectrum

Post by cj7hawk »

It might be easier to build a small PCB to buffer the bus for any peripheral... Then you could also test your theory on other devices.

You could also include a short length of ribbon cable to an edge connector so that other peripherals didn't have to fit the toastrack's tilt.

Don't forget the clock issue with the weak clock signal on the 128s as well as they only use a transistor to drive the clock... That needs buffering as well. Though ideally it should be buffered before it gets to the CPU to avoid propagation delay in the clock.
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