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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. Serial communication also has a deterministic behaviour to avoid collisions of data packets, making it more reliable for a linkage system with many devices. Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. For example, Belden 3105A cable can be specified, but different types of cable with equivalent characteristics can be used. These characteristics include: definition of a unit load, voltage ranges, open-circuit voltages, thresholds, and transient tolerance. This auto transmit unit features Bi-directional serial conversion from RS232 to RS485 in 2 wire half-duplex communications mode. Jumper J10 will place the unit in Loop-back mode for self test.

A special jumper(J9) can further shorten the turn-around time to accommodate fast responding target devices. It is commonly implemented to control video surveillance systems and link security control panels to devices such as access control card readers. 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. Alarm systems: Rs485 cables in alarm systems help transfer alarm signals and information from alarm devices to the control center. This has been done to avoid reflections of signals. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. This ensures that all security-related information and incidents are transmitted quickly and with high reliability. The flow meters relay flow information in real-time to the central control system via RS-485, halting the supply once the specified amount of reagent is dispensed. The maximum number of repeaters that should be serially connected is 3. A higher number introduces excessive delays in the communication system. 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.

The RS485 network must be designed as one line with multiple drops, not as a star. There is a whole plethora of SenseCAP industrial-grade sensors, and this is just one of the many! This allowed users in manufacturing facilities to connect larger applications and whole machines with one protocol. A Modbus RS485 connects a Master device to one or more Slave devices. Modbus devices communicate using a Master-Slave technique where only one device (the Master) can initiate transactions (AKA queries). 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). Manufacturing facilities use serial communication to link their devices together. In this method, all nodes within the network share the same pair of communication lines. Setup is the same when multiple meters are interconnected via cables CBL03, except that all meters now need to be set to a different digital address via their front panel. First set jumpers for your type of communications. The cable to be used is a shielded twisted pair (telephone type). Specify type of power connector when ordering. A polarity protected linear power regulator accepts supply voltages from 8 to 15 VDC, at 0.5 Watts max.

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. Electric noise can cause all kinds of problems for electronics. It can drive things 1200 meters, which RS - 232 is about 15 meters maximum. The branches must be no longer than 1200 m! The end resistance must be used only at the ends of the main cable. In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. 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. 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).

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