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. The branches must be no longer than 1200 m! All the ‘A’ terminals must be connected together and all the ‘B’ terminals must be connected together respectively. In order to avoid errors when many devices are connected, cables of the same colour should be used for all the connections to the terminals A and cables of the same colour should be used for all the connections to the terminals B of the various devices (e.g. white for A and blue for B). This makes it easier to identify cabling errors. The cabling of the industrial communication systems (Modbus RS485) is different in some ways from the cabling used for power cabling and the electrician may experience some difficulties if he is not an expert in Modbus communication networks. Henceforth, we shall consider Slave devices to be measuring instruments with serial communication, even if the cabling is similar for all Modbus devices.
Interoperability of even similar devices from different manufacturers is not assured by compliance with the signal levels alone. The two values provide a sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. 12 V, i.e. ±7 V on top of the 0-5 V signal range. 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. Utilizing a series of dedicated interface devices, it allows PCs and industrial controllers to communicate. 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 standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting the signal reference common and equipment case grounds. 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.
PC environment. 8P8C modular connectors are used in this case. 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. 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. For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. Because a mark (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage and space (logic 0) represented with a positive one, A may be considered the non-inverting signal and B as inverting. Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. RS-485, like RS-422, can be made full-duplex by using four wires. 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. By using repeaters very large RS-485 networks can be formed. Digital communications networks implementing the standard can be used effectively over long distances and in electrically noisy environments. The EIA has officially disbanded and the standard is now maintained by the TIA as TIA-485, but engineers and applications guides continue to use the RS-485 designation. Let’s talk about what we now know about RS485. 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. The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). 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. You may be wondering, what can I do with serial communications, especially RS485. The greatest feature of RS485 is the fact that you can connect up to 32 devices on one system.
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