For higher speeds and longer lines, the termination resistances are necessary on both ends of the line to eliminate reflections.Use 100 Ω resistors on both ends. Ideally, the two ends of the cable will have a termination resistor connected across the two wires and two powered resistors to bias the lines apart when the lines are not being driven. Address, Function, Data, and LRC are ASCII hexadecimal encoded values, whereby 8-bit values (0-255) are encoded as two human-readable ASCII characters from the ranges 0-9 and A-F. This helps to minimize the impact of electromagnetic interference, causing the voltage between the two wires to deviate. 5 Volt supply by a dual RS232 driver chip that has a built-in charge pump voltage multiplier. We also see that the maximum allowed voltage levels for all interfaces are in the same range, but that the signal level is lower for the faster interfaces.Because of this RS485 and the others can be used in situations with a severe ground level shift of several volts, where at the same time high bit rates are possible because the transition between logical 0 and logical 1 is only a few hundred millivolts.Interesting is, that RS232 is the only interface capable of full duplex communication.

RS232 has a separate communication line for transmitting and receiving which-with a well written protocol-allows higher effective data rates at the same bit rate than the other interfaces. RS485 is used as the electrical layer for many well known interface standards, including Profibus and Modbus . To support Modbus communication on a network, many modems and gateways incorporate proprietary designs (refer to the diagram: Architecture of a network for Modbus communication). When there is no data activity on an RS485 network (or in many instances RS422 networks, other than point-to-point), the communications lines are "floating" and, thus susceptible to external noise or interference. Because of this, the resulting noise current is many factors lower than with an ordinary straight cable.Shielding-which is a common method to prevent noise in RS232 lines-tries to keep hostile magnetic fields away from the signal lines. Modbus defines a protocol data unit (PDU) independently to its lower layer protocols in its protocol stack. The mapping of MODBUS protocol on specific buses or network requires some additional fields, which are defined as application data unit (ADU).

Modbus is often used to connect a plant/system supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. For example, when a Microcontroller unit (MCU) connects to a sensor to read its data by Modbus on a wired network, e.g RS485 bus, the MCU in this context is the client and the sensor is the server. Modbus supports communication to and from multiple devices connected to the same cable or Ethernet network. In the straight cable, all noise current is flowing in the same direction, practically generating a looping current just like in an ordinary transformer.When the cable is twisted, we see that in some parts of the signal lines the direction of the noise current is the oposite from the current in other parts of the cable. With RS485 communication distances of 1200 m are possible.Differential signal lines also allow higher bit rates than possible with non-differential connections.

Since it uses a differential line over twisted pair (like EIA-422), it can span relatively large distances (up to 4000 feet or just over 1200 metres). If high noise immunity is needed, often a combination of twisting and shielding is used as for example in STP, shielded twisted pair and FTP, foiled twisted pair networking cables.Differential signals and twisting allows RS485 to communicate over much longer communication distances than achievable with RS232. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. RS485 provides a means of interconnecting multiple devices in a series using a twisted pair of wires to facilitate data exchange. Because matters of protocol are left to the user, it is often difficult (if not impossible) to connect RS485 devices from different manufacturers on the same network. ADU is formed by a client inside a Modbus network when the client initiates a transaction.

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