Internal ram upgrade to 640k - continued

[1024MAK]

Is decoding of the FC2,FC1,FC0 signals really required in a QL. After all, to a 68000 series CPU, a QL is a simple device?

Mark

[TabajaraLabs]

So, what is going on here?

The 640K internal expansion is SO simple and "well documented"

This is part of the QubIDE schematics. Just connect a 512KByte SRAM chip to the bus and the 16V8 GAL code is here:

Ampliacion.zip

(754 Bytes) Downloaded 201 times

It works, I did that in my QL and it is working perfectly.

Anything else?

Greetz from Brazil

[1024MAK]

Also the schematic for the Miracle Expanderam 512K does not use the FC2,FC1,FC0 signals.
Schematic available from http://www.dilwyn.me.uk/docs/hardware/QLperip.zip file name "expram_sch.pdf".

Mark

[Dave]

On the QL, there can be 'harm' from using a faster chip.

The QL struggles greatly with noise, and a fast transitioning GAL can cause undesired inductances. At QL speeds, you'll never need faster than -15 parts. -20 parts are fine in every general purpose situation on the QL bus. -7 parts are often a bag of hurt unless you're an experienced PCB designer.

[TabajaraLabs]

On the QL, there can be 'harm' from using a faster chip.

The QL struggles greatly with noise, and a fast transitioning GAL can cause undesired inductances. At QL speeds, you'll never need faster than -15 parts. -20 parts are fine in every general purpose situation on the QL bus. -7 parts are often a bag of hurt unless you're an experienced PCB designer.

Eh...Dave, a -7 part won't work faster in place of a -15 or -20 part...

[Nasta]

On the QL, there can be 'harm' from using a faster chip.

The QL struggles greatly with noise, and a fast transitioning GAL can cause undesired inductances. At QL speeds, you'll never need faster than -15 parts. -20 parts are fine in every general purpose situation on the QL bus. -7 parts are often a bag of hurt unless you're an experienced PCB designer.

Eh...Dave, a -7 part won't work faster in place of a -15 or -20 part...

That's where you are mistaken. It won't work faster in terms of the frequency but faster in terms of edge speeds and that's what counts. Digital actually only works during edges, keep that in mind.

[Dave]

Hehe, Brane2 beat me to it, but yes, there's a lot of difference.

It's compounded though. The QL has noisy signals, and the signals are just noisy enough that even a small spike, echo, resonance or inducted signal can cause a -7 or -10 GAL to change state when it's not due. The transitions are so fast, they drive lines in the poor QL mainboard and the induced noise in nearby lines is..... a show stopper.

[1024MAK]

Of course, there are a number of tricks to slow down a fast switching signal...

Using series resistors is the most common method. But as was said before, the 7.5 MHz QL is not fast, so the slower GAL chips are fine.

Oh, and what we are really talking about here, with "fast edges" is slew rate. The higher the slew rate, the faster the signal level goes from low to high (and high to low). A higher slew rate requires a wire or PCB trace to be treated as a "transmission line", that is required to handle a very high bandwidth, high frequency square wave signal.

Mark

[tcat]

For the relatively simple job that the GAL is doing, and it will not be operating with any really high speed cycles, the current consumption should be at the lower end.
As such, using a 7805 voltage regulator should be okay. Fit it flat to the board and put a large area of copper track around the tab area and you should not need a metal heatsink.
Mark

Hi Mark,

I put some ceremic capacitors around 7805, I want to also put an elyt smoothing capacitor on the +5V output of 7805.
What value you would recommend to be?

Many thanks
TCAT

[Dave]

You'll only need a 250mA or 500mA 7805 - a 1 or 1.5A 7805 is overkill and will just make a lot of heat.

On this 500mA 7805 in SuperRAM I have used 2x .1uF ceramic and 2x 20uF 16V electrolytic, one each input to ground and output to ground. I also placed a 0.1uF ceramic at each IC, as close to the +ve pin as possible.