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100 ohms, Cat 5 cables almost meets the electrical requirements for RS-485 cables. The capacitance is close to 12.5 pF/foot but there is a small impedance mismatch (100 ohms for Cat 5, 120 ohms for RS-485). End of line termination resistor (120 ohm 1/4 watt) to be installed at both ends of network. The end resistance must be used only at the ends of the main cable. For example, Belden 3105A cable can be specified, but different types of cable with equivalent characteristics can be used. The maximum number of devices that can be connected to a main cable is 32, including the Master. 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. Bias termination - Using resistive termination decreases noise immunity, particularly if the cable is loaded with many devices. Shield on communication cable is to remain continuous and ground only at the controller. The maximum number of repeaters that should be serially connected is 3. A higher number introduces excessive delays in the communication system. In order to increase the extent of the Modbus network, repeaters can be used; and signal amplifying and regenerating devices provided with two communication ports that transfer to each what they receive from the other. 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). Henceforth, we shall consider Slave devices to be measuring instruments with serial communication, even if the cabling is similar for all Modbus devices. The serial output at the UART idles at the logic high (mark) level.

With careful design, many peripherals can communicate via the SPI, and powerful multi-processor systems can be linked using this high speed bus. 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. 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. Because differential signals have inherently better signal-to-noise properties, reliable RS422 communications can be sent over much longer distances compared to RS232. The main cable must be no longer than 700 m! There are many categories of Ethernet cables we can use like CAT-4, CAT-5, CAT-5E, CAT-6, CAT-6A, etc. In our tutorial, we are going to use CAT-6E cable which has 4 twisted pairs of 24AWG wires and can support up to 600MHz. It is terminated at both ends by an RJ45 connector. In this case, cable connections may be made to Serial 1 on either the 10-pin Serial Communications Header or the Serial 1 Connector.

It seems if there is a lot of RFI in the installation, shielded twisted cable can be justified. You can use one or both of the PDQ Board’s RS485 links to create such a multi-drop serial network. Single master systems (many other communications schemes are available) offer a straight forward and simple means of avoiding "data collisions" in a typical 2-wire, half-duplex, multi-drop system. For this reason, frame-level cyclic redundancy checks are much more widely used for validating data from serial links, rs485 cable network connections and storage media. There are three flag bits implemented in the SPSR (SPI status register). Like RS232, the data bits are transmitted in reverse order, with the least significant bit transmitted first. Handles all serial data on a bit by bit basis. Longer branches could cause signal reflections and generate disturbances and consequent errors in the reception of data. The branches must be no longer than 1200 m! This distance does not include the branches (which must nevertheless be short). The cable shield must be earthed only in one point.