Query SIO Speed Improvements

[Wiffleplop]

I've been reading through Lotharek's pages after picking up one of his SIO2SD cards, and an SIO cable from a friendly forumer with a shop over the channel. That's been hacked in half, with one hald used to connect to the five pins on the SIO2SD unit, the other kept as spare or some such, in the future.

 

There's a bit of text that talks about improving the speed of the SIO port here to 127kb/s by adding a 4.7k ohm resistor between pins 1 and 5 on the port, and removing the two capacitors C77 & 78. That's the bit that's confusing slightly, as my knowledge of the the 800XL's internals is zero at the moment, and my electronics knowledge is minimal. When he talks about removing the caps, do you replace them with a short loop of wire, or just de-solder and leave their holes open?

 

Has anyone tried this, and what were your results? Apparently you need some enhanced SIO routines, which you can find here, but I've yet to delve into how to get that running.

 

basically, I'm looking for a little hand-holding and advice on the process. I'm not scared of soldering, or sourcing parts, just of doing it wrong!

[Rybags]

The capacitors advised to be removed - take them out, do nothing else there. IIRC they were only ever included as some noise reduction method.

 

The 4K7 resistor - I imagine that is to improve the transition time from 0 to 1 on SIO OUT.

Article says "if you don't have one already" - I'm not sure if any machines have it by default.

 

The operative thing though is the capacitor removal where they exist. Even at lower speeds they're known to cause problems.

 

And yes, best operation at the very highest speeds is generally with custom routines. The default method of IRQ-based SIO is only good for the lower turbo speeds, polled SIO works best for the very highest ones.

[Bryan]

It's all about increasing slew rate (change in voltage/time). Capacitors can be used to decrease the speed of transitions by absorbing the current of a changing signal. Removing the capacitors will make the signal faster, but sometimes this isn't desired as it leads to more radiated RF and things like overshoot and standing waves, etc... These become very problematic at MHz-GHz frequencies but Atari's engineers were probably just being cautious and planning on low SIO speeds. The pull-up resistor is to help combat the asymmetrical nature of the signal (drops quickly, rises slowly).

 

So, leave the caps out, and add the resistor. This mod would be useful for Happy users who are having ultraspeed issues.

[Wiffleplop]

Thanks fellas. I have my resistor ready and might get to crack the case (I suspect for the first time in its long life) later on today. We shall see how that goes

[Roydea6]

Thanks fellas. I have my resistor ready and might get to crack the case (I suspect for the first time in its long life) later on today. We shall see how that goes

Read this post and thread... about the resistor..

 

http://atariage.com/forums/topic/222061-improving-sio-speed-for-sio2sd/?p=2937746

[Wiffleplop]

Pins 1 and 5 of the SIO2SD board? I did read that right, huh?