Defines electrical input and output characteristics of intrinsically safe transmitting and receiving circuits forming a low speed digital data transmission link in which data is transmitted using current loop techniques.

This Part of BS 6556 specifies the electrical input and output characteristics for the optical coupling of intrinsically safe transmitting and receiving circuits for use in coal mines. These form a low speed digital data transmission link in which data is transmitted using current loop techniques.

A data transmission system comprises a master and up to eight physical slaves so arranged that the transmission line is energized only from the master and galvanic isolation is achieved by optical coupling at each slave.

NOTE 1 Each physical slave may support more than one addressable logical slave as specified in Part 3 of this standard up to a maximum of 15 addressable slaves for the complete system.

This Part of BS 6556 only applies to apparatus where the circuits connected to the transmission lines are designed to be intrinsically safe in accordance with BS 1259, BS 5501-7 or BS 5501-9 as appropriate.

NOTE 2 Where apparatus, which is intended to be used in potentially explosive atmospheres, contains other circuits which are not intrinsically safe, those circuits will have to be given an alternative form of protection in accordance with BS 229, BS 4683 or BS 5501 as appropriate.

NOTE 3 The titles of the publications referred to in this standard are listed on the inside back cover.