In general, the electrical characteristics specified in this Recommendation apply to interchange circuits operating at data signalling rates up to 75 bit/s, which have been covered in Recommendation V.31, and also to interchange circuits operating at data signalling rates up to 1200 bit/s. The electrical characteristics are specified to provide compatibility with existing equipment according to Recommendation V.31 which meet the voltage and current values given in Table 1/V.31 bis. The resistance values defined in Recommendation V.31 are converted into current and voltage values to meet the optocouplers requirements. Each interchange circuit consists of two conductors (go and return leads) which are electrically insulated from each other and from all other interchange circuits. A common return lead can be assigned to several interchange circuits of a group.
ITU-T V.31 BIS SPANISH-1988 history
1988ITU-T V.31 BIS FRENCH-1988 ELECTRICAL CHARACTERISTICS FOR SINGLE-CURRENT INTERCHANGE CIRCUITS USING OPTOCOUPLERS