Re: shadow memory

"Arne Chr. Jorgensen" wrote:

> Patrick Huss:
> Index pulses: I asked about this.
>
> Here is why:
>
> The encoder index, may not be used. If I am not wrong, I came across
> a system were the encoder index signal, - captured the count
> registers on an encoder -"decoder" chip. - to another register set.
> In such a case, you could have used the index during a probing on a
> surface. Fred said that it would need hardware to capture this
> counter.
> I have just checked the Stg manual, etc. - and the decoder chip
> don't have this feature, but it will generate an interrupt. If you
> could act on this interrupt, and then read the counter registers, -
> then maybe it could be useful after all.

I'm sorry, but you're wrong. The LS7166 chip from US Digital
DOES have a facility to read the index pulse, and use it, you just
have to do things a certain way, and some of the 'glue' logic on the
STG board takes care of it. There is an external I/O pin on the 7166
that latches the 3-byte count so it can be read byte by byte without
any updates while you are reading it. This pin is usually flipped
at the time the interrupt is given to the CPU, so that all n channels
are sampled simultaneously. When you are setting up for a home
operation, however, you switch this pin to be controlled by the
index signal from the encoder, and you wait until the index pulse
has been seen. Now, the ouput latch contains the exact count at
the time the index pulse ocurred!

Anyway, EMC DOES use this feature. While I haven't got the
coarse switches installed on my machine, I know others have
hooked them up, and it does work for them.

Jon