At least one three phase driven pumps (56) in a hydrocarbon well (10) well
bore (12) uses frequency shift keyed digital signals (78) to pass
instructions from a surface station (46, 26, 34, 30) to a tool (14, 76,
56) and reports from a tool (14, 76, 56) to the surface station (46, 26,
34, 30) in common mode through the three phase cables (16) using a star
point (58) of the three phase cables (16) after each pump as the signal
transmission and reception point and as the point for coupling a DC Power
supply (28), also provided in common mode on the three phase cables (16),
to the down hole electronic equipment (72 76). The surface equipment (46)
and the tool (14) use as a common ground. The digital messages comprises
an address portion (80) identifying either the tool (14) for which an
instruction (82) is destined or the tool (14) from which a report (also
82) originates. The digital message (18) also comprises a checksum (84)
for error identification and correction.