> I see no logical reason for the behaviour you are seeing

That makes two of us. This is about as succinctly as I can put it

http://home.clear.net.nz/pages/joecolquitt/SX_pins.html

Amongst other things I've checked Vcc and Mclr for glitches

Note that the first analyser picture shows correct operation. Throw
another pin excursion into the mix though..........

There seems to be no apparent consistency. I never know what
to expect, which tends to cramp one's style. Some days the damn
thing makes me feel like a complete muppet

Handshaking between the two micros works fine. As the SX is
running 54x faster than the PIC I'm using INTF to store the Ack
coming from the SX. The code snippet to do this is in the diagram
showing the inter-connection

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