A data communications system and method for transmitting data over a string between a surface location and a sub-surface location in a well bore in which a load varying device at the sub-surface varies the mechanical load on the string to be indicative of the data and a load measuring apparatus at surface monitors the mechanical load on the string and decodes the data. Data transmission is described from a pump assembly through a sucker rod string. Embodiments of load varying devices using electrical generators, friction rollers and hydraulic and pneumatic brakes are also described.