It requires minimal wiring and can share the wiring among several seats, reducing weight. 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 configuration and specifications of RS485 make it faster and extend the range of data transmission. The foreword to the standard references The Telecommunications Systems Bulletin TSB-89 which contains application guidelines, including data signaling rate vs. 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. 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.
A simple network of a PLC, VFD, and an HMI allows remote control of motors in an industrial setting. Multiple receivers may be connected to such a network in a linear, multidrop bus. 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. This way, the lines will be biased to known voltages and nodes will not interpret the noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered. Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. Under some conditions it can be used up to data transmission speeds of 64 Mbit/s. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors.
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. A Modbus RS485 connects a Master device to one or more Slave devices. RS232 could only handle one. RS232 could only handle one at a time. RS485 can handle up to 32 connected devices. Think of RS485 as the younger, faster brother of RS232. Just like it’s older brother RS232, RS485 is a form of serial communication. Another plus for RS485 is that it is less susceptible to noise issues. Termination resistors also reduce electrical noise sensitivity due to the lower impedance. Electric noise can cause all kinds of problems for electronics. By using repeaters very large RS-485 networks can be formed. RS-485 does not define a communication protocol; merely an electrical interface. Utilizing a series of dedicated interface devices, it allows PCs and industrial controllers to communicate. 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. Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system.
The maximum number of repeaters that should be serially connected is 3. A higher number introduces excessive delays in the communication system. This helped RS485 become a widely used and very versatile form of serial communication. It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. The EIA has officially disbanded and the standard is now maintained by the TIA as TIA-485, rs485 cable but engineers and applications guides continue to use the RS-485 designation. This allowed users in manufacturing facilities to connect larger applications and whole machines with one protocol. One very common example in the automation world is remote control of VFDs or Variable Frequency Drives. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. All in all, RS485 really allowed full automation possible with its ability to connect so many devices at once.
