Thanks again Bill, I'll check the speed and try the pullup.

The Zx reply was quite easy to work out :) As you can see in from the two sample replies in my previous post, Rx: appears as 0018 3A and Ix, appears as 0918 08. 'x' appears consistent, returning 18h, ':' is 3A which is correct, ',' appears as 08h.

Consistent, by this I mean that consecutive replies to a given request are often the same, or they fall into a pattern. Over time the values and patterns do change, with the possible exception of the ':'. Looks like a baud rate or level issue, or my Prolific is marginal.

No scale connected until I stop messing around and verify 1.9v operation, too costly to risk. What the PIC thinks it can see is another matter, stroking the component side of the board is enough to fool it into thinking something is there.

Can you confirm the value of the LED series resistor in the 1.5v supply part of the circuit ?