EchoPilot log/depth giving strange readings?

I have an EchoPilot log/depth. The speed reads a constant 18.3 knots even when the yacht is on it's moorings. Also the depth reads a constant zero.

If I disconnect the depth sensor the speed reads zero. If I disconnect the log sensor the speed reads zero.
Any suggestions would be happily received.
Thanks.

The depth indicates a constant zero.
I have confirmed the supply is ok. from a seperate source. The instrument illumunation is lit.

John
July 2011