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The most common cable type is Cat 5e-UTP (unshielded twisted pair) which may work over shorter distances in less demanding applications with low EMI noise levels. Rs485 is low impedance balanced , so twisting the pair helps with noise immunity as well as reducing the attenuation effect of higher baud rates due to the parallel capacitance of the wires in the cable jacket over the long lengths. With a typical capacitance of 15 pF/ft. Utilizing a series of dedicated interface devices, it allows PCs and industrial controllers to communicate. These are used in programmable logic controllers and on factory floors. The issue also exists in programmable logic controller applications. In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. And you should never use the shield as a third conductor. Also, when some devices on the RS485 are NOT opto-isolated, the third wire becomes essential to avoid the units being damaged. Be sure to use a wire pair such as the blue and blue/white pair for the data lines and a third wire or pair twisted together for the common connection. We use 2 pair telephone cable for RS485 communication up to 1200 metres.

The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents - enough to melt signal cables, PCB traces, and transceiver devices. Termination resistors also reduce electrical noise sensitivity due to the lower impedance. And even worse, if you have a non opto-isolated device installed on that network, any noise could "drain" to ground through the device itself instead of flowing through the ground terminal at the end of the line, damaging the device in the process! The shield works as a "drain" for any noise that could be picked up by the RS485 network and "drain" it to ground. This works and is sometimes also recommended by many vendors. The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, i.e. a line or bus, not a star, ring, or multiply connected network. The EIA once labeled all its standards with the prefix "RS" (Recommended Standard), but the EIA-TIA officially replaced "RS" with "EIA/TIA" to help identify the origin of its standards.

RS-422 and RS-485 Standards Overview and System Configurations, Application Report (pdf). RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. By using repeaters very large RS-485 networks can be formed. Because they are produced in such large quantities, Cat 5 cables are relatively inexpensive, often less than half the price of specialty RS-485 cabling. All communication cabling to be installed in a daisy chain configuration, no T’s are permitted. RS-485 does not define a communication protocol; merely an electrical interface. It also defines three generator interface points (signal lines); A, B and C. The data is transmitted on A and B. C is a ground reference. The termination also includes pull up and pull down resistors to establish fail-safe bias for each data wire for the case when the lines are not being driven by any device. It is important to know how to wire devices on an RS485 network, especially when a non opto-isolated device is present.

In fact, there is no such thing as an RS485 cable since RS485 is a protocol for serial communication (like between inverter and Zero export device) and Ethernet is a protocol for carrying internet data (between router and inverter) whereas CAT6 is a cable standard. RS-485, like RS-422, can be made full-duplex by using four wires. SENSEWAY connects to WIRED PROs using custom-sized RS485 cables and a T connector. TSB-89A, Application Guidelines for TIA/EIA-485-A does not recommend using star topology. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans. RS-485 only specifies the electrical characteristics of the generator and the receiver: the physical layer. Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. These characteristics make RS-485 useful in industrial control systems and similar applications. These characteristics include: definition of a unit load, voltage ranges, open-circuit voltages, thresholds, and transient tolerance.