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Just like it’s older brother RS232, RS485 is a form of serial communication. The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible. In this case it is possible to connect the main inlet and outlet terminal directly to the terminals of an instrument without creating a branch. Each device has a communication port with two terminals, which are indicated for the sake of convenience as A and B. In these two terminals the communication cable is connected so that all the devices that take part in the communication are connected in parallel. It is important to know how to wire devices on an RS485 network, especially when a non opto-isolated device is present. Now there was certain instances I used STP wire for rs485 because I had a lot of devices connected in a long run chain (DMX512 lighting fixtures). In practice, Cat 5 cables have been used successfully in many installations, but there are some concerns. 24 AWG solid wires, which are typically used in Cat 5 cables, can break easily when connected to a screw terminal block, which does not provide any strain relief.

PC environment. 8P8C modular connectors are used in this case. But You stated you are going to install this at a commercial setting, so it always better to use the worst case environment situations, and use shielded cable. No matter the devices, it is always good on real site installation to use a shielded cable, with 18-22AWG wiring and the shield connected ONLY to one end of the line. Utilizing a series of dedicated interface devices, it allows PCs and industrial controllers to communicate. For an on, space or logic 0 state, the driver's A terminal is positive relative to the B terminal. The issue also exists in programmable logic controller applications. 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. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. Access control systems: Rs485 cables are mainly used to connect and transmit data between access control devices such as security gates, RFID cards, or time attendance machines and the control center.

In some countries, inserting two cables into the same screw terminal is permitted. RTU (Remote Terminal Unit) mode transmits 8-bit messages as two 4-bit hexadecimal characters in a continuous stream. This helps to minimize the impact of electromagnetic interference, causing the voltage between the two wires to deviate. RS-485, like RS-422, can be made full-duplex by using four wires. This includes speed control using Variable Frequency Drives (VFD) or inverters, as well as controlling simple PLC and HMI network systems. It may be used to control video surveillance systems or to interconnect security control panels and devices such as access control card readers. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. The Modbus protocol further enables RS485 by allowing a master to send and receive data from specific sensors on the bus or to communicate with all the sensors simultaneously. The configuration and specifications of RS485 make it faster and extend the range of data transmission. 2 wires is all you need to make it work.

Polarity of the wires must be observed. The main cable must be no longer than 700 m! The maximum number of devices that can be connected to a main cable is 32, including the Master. Rs485 cables not only vary in the structure of the core but also in the number of twisted pairs. The cable recommended in this type of connection is the Belden 9842, which has 2 twisted pairs. The cable to be used is a shielded twisted pair (telephone type). 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. The WattNode® BACnet and Modbus® meters communicate using RS-485 (TIA/EIA-485-A) half-duplex, differential signaling at 9600, 19200, 38400, and 76,800 baud. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422.

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