Hello 0010 all,

First some data on the 8271 from Intel's Handbook (see also in attachments):

1. For the clock at pin 3 of the 8271 4MHz is the only frequency mentioned
2. DACK and CS simultaneous being zero is explicitly stated "Not Allowed"
3. "For non-DMA transfers, this signal (DACK) should be driven in the manner of a "Chip Select"."

Now here are my questions:

1. a. The Atom and Beeb use 2MHz, why the difference ❓
   b. Must the FDC clock signal be derived from the µp clock generator or may the FDC have a separate crystal oscillator ❓
2. Is that "Not Allowed" not exactly what the Atom and Beeb do ❓ (see diagram below)
3. Is this "Chip Select" how the Atom and Beeb use DACK and why it is pulled low together with CS ❓

Although this forum is called 6502.org I hope someone can help me out on the questions I have concerning the way in which Intel's 8271 FDC is used by Acorn.

_________________
Gr tings, Louis

May your wires be long and your nerves be strong !

It is all about the 6502 and we often discuss many old systems as well as our retro-new ones.

Have you looked at/Do you know about stardot though? It's highly likely there are more folks there who can help you.

https://stardot.org.uk/forums/

-Gordon

_________________
--
Gordon Henderson.
See my Ruby 6502 and 65816 SBC projects here: https://projects.drogon.net/ruby/

According to the Beeb circuit diagram, the 8271 clock is 4MHz.
see perhaps
https://stardot.org.uk/forums/viewtopic.php?t=17583
or
https://mdfs.net/Info/Comp/BBC/Circuits/BBC/bbc.gif

Hello Gordon and Ed,


I started a few threads on Stardot about the 8271 long before I registered here. Little reaction, especially about the questions I also posted here almost non. I think my questions are too specific to the 8271, nobody uses the 8271 any more and I fear interest in it has waned.

I also posted some at two sites where the de-cap of the 8271 is discussed, no reaction at all.

That's why I started this thread here . . .


In the Beeb diagram that I have there is an LS393 (IC86) that gets 16MHz or 8MHz (via link S27), the LS393 divides this by four (output QB), thus pin 3 of the 8271 can get 4MHz or 2MHz.

Apart from that, on the Atom FDC card the 8271 solely receives 2MHz on pin 3, which is the basis of my first question about the frequency.

_________________
Gr tings, Louis

May your wires be long and your nerves be strong !

Perhaps useful to read
A Hardware Guide For The BBC Microcomputer where page 30 talks about S27 and the clocking of the FDC.