I've hit on an odd problem with the equ's at the start of my code

I have various ones setting up register addresses etc, eg

data_in equ     21h
data_out        equ     22h

But when I try

ack_data        equ     FFh

I get an Error 113, undefined symbol.

Are the others not being translated as hex?  If so, are the data registers
actually at 21 decimal?  This should cause horrible crashes as I'm using
these registers already...

If I use

ack_data        equ     0xFF

it seems OK

What am I missing?

Nigel