When planning to implement a communication system (such as a sub-metering system), attention needs to be paid to the structure of cabling, since it is an essential part of the communications structure. Having a planned and organized strategy is crucial to the installation of a sub-metering system. Your cabling system is like a network of roads that “transports” the data from devices to end-users, so it’s important to avoid the “bumps” and “holes” to provide a smooth data journey.
The basis of a well-implemented cable structure is the right choice of cables. Once you get that right, you will get the performance you need and expect.
Modbus RS-485
The standard that defines the electrical characteristics of receivers and drivers for use in serial communications systems is the RS-485 cable. It supports multipoint systems and has balanced electrical signaling. RS-485 cables use the same differential signaling over a twisted pair like RS-422 to support multidrop communication links and local networks. They can be used at distances up to 4,000 ft (1,200 m) and at lower speeds or data rates up to 10 Mbit/s.
The RS-485 cables can be installed for full-duplex communication. In full-duplex communication, components on both sides can receive and transmit information simultaneously between each other. With half-duplex systems, the reception/transmission of data must happen alternately – while one point is receiving, the other must only transmit. Full-duplex communication requires 4-wire cables, while half-duplex requires only 2 wires. As for sub-metering, half-duplex communication should do.
Repeaters are used to increase the extent of the Modbus network. However, repeaters can create issues in data transmission or reception and it is ideal to avoid them.
CAT 5e and CAT 6
Cat5e and Cat6 allow the use of the other twisted pairs in the cable for spares, to run a return to the daisy chain, and to run power to a sub-metering device if needed. If the impedance doesn’t match, then a 120 Ohm termination resistor at the end of the daisy chain will be required. The goal of impedance matching is to minimize signal reflection or maximize power transfer to the load. Without a terminating resistor between the two wires, the signal can be overwhelmed by noise.
Cables that meet Cat6 specifications are found to provide significantly lower near-end crosstalk (NEXT) or interference in the transmission than other cables. It also improves ELFEXT (equal level far-end crosstalk), insertion loss, and return loss when compared to Cat5e. This results in higher data rates, fewer errors, and less noise in signal transmission.
Cat5e and Cat6 cables can be used for lengths up to 100 meters. The benefits of these cables are the option of being terminated in an RJ-45 connector and speeding up installation time.
When installing a sub-metering system, Cat5e and Cat6 are superior to your standard twisted pair commonly used in RS-485 chains in many ways. Cat5e and Cat6 provide a fast and solid signal transmission with spare pairs to share.
