My problem is solved !!! :-)

When using the AND instruction to mask the offset (and size) of the buffer the following happened:

Offset = 63
inc offset
offset = offset and 63 (64&63 = 0)

and '0' as an index for an array is prohibited in Bascom. The first valid index = 1
So TX_Buffer(0) was placed in SBUF which contained everything but the wanted character...........

So my buffer started at 1 and ended at 64. My offset started at 0 and stopped at 63.

I replaced the 'AND' instruction with a simple >=65? Offset = 1.

Peter