A Modbus RS485 connects a Master device to one or more Slave devices. A typical RS485 network can operate properly in the presence of reasonable ground differential voltages, withstand driver contentious situations, provide reliable communications in electrically noisy environments (good common mode rejection using twisted pair cable, shielding provides additional protection), and support thirty-two or more (many IC manufacturers have 1/2, 1/4, 1/8 unit load devices) drivers and receivers on the line. An "Automatic Converter" (RS232 to RS485) will turn on the RS485 transmitter when data is detected on the RS232 port, and revert back into the receive mode after a character has been sent. The combination of the cable impedances and/or termination resistors will load the network and can make communications unreliable. Twisted pair wire with a characteristic impedance of 120 ohms is recommended with 120 ohm termination at each end of the communications line. In general RS485 is designed for multi-drop, "daisy-chain" operation over a single twisted pair cable with a nominal characteristic impedance of 120 Ohms. The receiver is able to respond to differential signal levels of 200mV over the common mode range. The receiver load impedence is 12K ohms (or higher) and transmitter "leakage" current is ±100µA (or less) in either the powered or unpowered state.
While RS422 is comparable to RS485, it is limited to unidirectional data traffic, and is terminated only on the end of the line opposite the transmitter. One transmitter and 10 receivers are allowed on a network, with a distance limit of 3600ft. RS422 was on the market prior to RS485; however, due to loading limitations, one of the best uses of RS422 is probably in point-to-point communications, such as RS232 extension cords. Converters in general can be used to change the electrical characteristice of one communications standard into another, what is rs485 cable to take advantage of the best properties of the alternate standard selected. Another major advantage is that there can be more than one slave in the connection. This means data can be transmitted in both directions to and fro devices one direction at a time. The /SS (active-low slave select) is typically used to enable data transfers by slave devices when it is active low. If the /SS pin of the master is an input and if a low input level is detected, the processor sets the MODF bit in the SPI status register a "mode fault" condition.
12V. A driver in the high impedence (off) state is able to remain in this state over the common mode range, whether power is applied or not. If multiple analog input meters or transmitters are digitally addressed in command mode on an RS485 line using the Modbus RTU protocol or the Custom ASCII protocol, the rate becomes about 2 readings per second. Grove sensors to MODBUS RS485 industrial-grade sensors. While shielding is not specified for RS485 systems, it can help in many instances. 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. This allows standard point-to-point full duplex communications, as well as a multi-drop configuration with one master (a single QVGA Controller or a desktop computer) and multiple QVGA Controller slaves. The recommended arrangement of the wires is as a connected series of point-to-point nodes, a line or bus.
When the receiver reads the voltage on the other end there could be as little as 0.2 volts difference between the two wires. In general it is very important not to run communications wires in the same trough or conduit or in parallel with AC power cables. 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. What made it such an advancement over RS232 was the fact that it could transmit not only a single device to device transmission, but also a communications bus to connect multiple devices at once. When full-duplex is converted into half-duplex only one device at a time can transmit data. This allows you to configure full duplex multi-drop networks in which a single master can sequentially address one of many slaves and start a full-duplex exchange of data.