Hi Erik,

No insult taken!

This has come up before and P35 of the data sheet specifically says you can run a two wire implementation by patching DIN/DOUT together

From the data sheet:
"An interface circuit between the AD7708/AD7718 and the
8XC51 microcontroller is shown in Figure 20. The diagram
shows the minimum number of interface connections with CS
on the AD7708/AD7718 hardwired low. In the case of the
8XC51 interface the minimum number of interconnects is just
two. In this scheme, the RDY bit of the Status Register is
monitored to determine when the Data Register is updated.
...
The 8XC51 is configured in its Mode 0 serial interface mode. Its serial interface contains a single data line. As a result, the DOUT and DIN pins of the AD7708/AD7718 should be connected together with a 10 kΩ pull-up resistor. The serial clock on the 8XC51 idles high between data transfers."

But as you suggested, I am going to try and use separate input and output channels, because I fear the DOUT is pulling the entire interface low and overriding the weak pull-up resistor on P3.

Its good to talk!

Thanks Erik!
Rgd,
Eoin