RS232 could only handle one. Another advantage over RS232 that I mentioned earlier is the number of connectable devices. Using a repeater, the main cable is divided into different segments, each of which can be up to 700 m in length and connect 32 devices (this number includes the repeaters). Rs485 cables not only vary in the structure of the core but also in the number of twisted pairs. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. The configuration and specifications of RS485 make it faster and extend the range of data transmission. These characteristics make RS-485 useful in industrial control systems and similar applications. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485.

In a computer system, SCSI-2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. This allows RS-485 to implement linear bus topologies using only two wires. RS-485 serves as a physical layer for the AES3 digital audio interconnect. Cable jacket: Made from high-quality PVC, the outermost jacket layer has high elasticity, helping to protect the cable from strong environmental impacts. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. 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. 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. Control and monitoring systems: A single pair of rs485 signal cable can connect up to 32 devices, with a signal transmission distance of up to 1200m. Therefore, using rs485 cables is an ideal choice for industrial control and monitoring systems, especially those with large-scale and high dispersion.

What made it such an advancement over RS232 was the fact that it could transmit not only a single device to device transmission, but also a communications bus to connect multiple devices at once. If on the other hand each terminal can accept only a single cable, a proper branch must be created using three auxiliary terminals for each instrument to be connected. Users can utilize a longer cable, up to 1200 meters or 4000 feet. The maximum RS485 cable length is typically listed as 1200 meters or about 4000 feet. The branches must be no longer than 1200 m! The main cable must be no longer than 700 m! As a rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 108. Thus a 50-meter cable should not signal faster than 2 Mbit/s. This includes speed control using Variable Frequency Drives (VFD) or inverters, as well as controlling simple PLC and HMI network systems. All of this allows you to speed up the VFD that controls the speed of that water pump. Let’s say you were monitoring a water pump filling a vessel. With your RS485 network, you can remotely monitor and control that pump from a control center.

PLC systems are used to monitor and control operations, while HMI network systems allow interaction with machinery and control them. The PLC is programmed to monitor the level of the water in the vessel. This is possible since the HMI is connected to your PLC via RS485. The PLC is also connected to the VFD via RS485. Think of RS485 as the younger, faster brother of RS232. RS232 could only handle one at a time. 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. This allowed users in manufacturing facilities to connect larger applications and whole machines with one protocol. However this contradicts the facts that an idle mark state is a logical one and the termination polarization puts B at a higher voltage in Profibus guidelines. Termination resistors also reduce electrical noise sensitivity due to the lower impedance. Ideally, the two ends of the cable will have a termination resistor connected across the two wires.