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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. 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. So take advantage of that and let your systems MCUs implement heart beat handshaking directly though the RS485 communications layer using appropriately designed small packets. 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. The maximum number of repeaters that should be serially connected is 3. A higher number introduces excessive delays in the communication system. The maximum RS485 cable length is typically listed as 1200 meters or about 4000 feet.

The maximum number of devices that can be connected to a main cable is 32, including the Master. Normally, this connection is made at one end of the main cable. Figure 1 shows an example of a correct Bus connection. A mode fault occurs when the SPI senses that a multimaster conflict (MC68HC11F1 Technical Data Manual, p.10-5) exists on the network as explained above in connection with the /SS input. If you are using the QScreen as a slave device and require the /SS signal for your external SPI hardware, configure one of the Port A pins on the Field Header as an input pin. These steps greatly reduce the chance that the communicating devices might be damaged by contention on the SPI bus. Rather, it relies on software handshaking via transmission of XON/XOFF characters to coordinate data transfer and ensure that information is not lost when one of the communicating parties is busy. For devices where you have to consider the devices at the far end of your cables to be unable to be modified, either hardware or software wise, you have to get clever regarding how you determine if the device is connected. Because the software UART is interrupt based, competing interrupts that prevent timely servicing of the Serial2 interrupts can cause communications errors on the secondary serial channel.

MODBUS over Serial Line protocol (2006). MODBUS over Serial Line Specification & Implementation guide V1.02 (PDF). End of line termination resistor (120 ohm 1/4 watt) to be installed at both ends of network. The recommended arrangement of the wires is as a connected series of point-to-point () nodes, a line or bus, Ideally, the two ends of the cable will have a termination resistor connected across the two wires. The DMX standard uses that RS485 (or, for clarity RS422 would be better) standard for its hardware medium description, but that's where it ends. They have Shielding Jacket over the insulation layer to protect against the Electromagnetic Interference and also each pair of wires is twisted together to prevent any current loop formation and thus much better protection against the noise. The far better situation is to realize that connected cables enable communications and disconnected cables impede communications. 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).

The shielding may be braided (be formed by a mesh of thin conducting wires) or be a foil (consisting of a sheet of metal wound around the conductors): the two types are equivalent. Most of the low-cost sensors and other modules like GPS, Bluetooth, RFID, ESP8266, etc. which are commonly used with Arduino, Raspberry Pi in the market uses UART TTL based communication because it only requires 2 wires TX(Transmitter) and RX (Receiver). The DMX standard is very strict: Each pair is unidirectional (like RS422), it uses longer mark and space times between datablocks than specified in computer Serial communication. But often when that happens it's more likely to be referred to as RS422 as this standard strictly defines those 2 separate pairs. Unlike what happens in many energy distribution systems, the manner in which the devices are connected in parallel is important. The data bits are also transmitted in reverse order, with the least significant bit transmitted first, after a start bit. Otherwise, dip switch 5 should be OFF so that it frees bit 3 of PORTA for use as general-purpose I/O. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485.

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