As far as possible, we try to use existing protocols and components that are generally available, such as serial interface circuits (USARTs). If the environment is very noisy, we use shielded twisted pair cables (STP). To make the connections less sensitive to noise, we use the +12V/-12V voltage levels of the RS 232 C standard.

Theoretically, it is possible to use the same twisted pair for transmission and reception, but the ordinary, cheap serial interface circuits don´t support this so it is better to devote one pair for transmission and another for reception. By using a master-slave type protocol, such as the normal response mode (NRM) in HDLC, we can use one branching cable instead of a separate pair for each device. We also have to provide for power delivery to the substations somehow. Unfortunately, the resistance of a standard twisted pair cable is too high for this purpose.

The interface circuit between the cable and a meter/control device might consist of a simple microcontroller and an AD- (or DA-) converter.