12-9 www.mo xa .c o m Multiport Serial Boards > The Basics of RS-232/422/485 Device Connectivity 12 The RS-422 and RS-485 protocols use differential transmission to achieve high-speed data transmission (up to 10 Mbps) over distances up to 4,000 feet (1.22 km). Differential transmission works by splitting each signal into two separate wires with opposite voltage states. The signals are subtracted at the receiving end, making this type of wiring configuration well-suited for noisy environments. RS-485 was designed for applications that require connecting multiple devices to a single data line. An RS-485 multi-drop network uses a balanced transmission system that can accept up to 32 devices on the same data line. This is achieved with tri-state drivers that are controlled by a programmable handshake line to ensure that only one device acts as a driver at any given time. In order to prevent signal reflection, termination resistors are used to match the impedance of the receive and transmit nodes. The resistance needed to match the characteristic impedance is specified by the cable manufacturer. The most common value is 120 ohms. Many of the devices used in today’s industrial environments are designed for the RS-422 and RS-485 interfaces, both of which use “differential transmission” to “subtract out” external electronic and electromagnetic disturbances. For this reason, RS-422/485 can be used to transmit data up to 1.2 km. In addition to the need for long distance and multi-drop transmission, many industrial applications also require isolation, proper housing, heavy-duty wiring, a reliable power supply, and over-surge protection. RS-422/485—tailor-made for industrial applications RS-422 Wiring RS-422 vs. RS-485 RS-485 Wiring Signal TxA TxB RxA RxB TxA TxB RxA RxB Signal Wiring 120 120 Signal DataA(-) DataB(+) DataA(-) DataB(+) Signal Wiring 120 120 TxD VCC 100K 1K R1 R2 1K 100K Receiver IC Board VCC V+ V- Standard Connection Type Operation Mode Drivers per Line Receivers per Line Max. Cable Length Max. Data Rate EIA RS-422 Full-duplex, Point-to-point Differential 1 10 4000 ft (1.22 km) 1 Mbps EIA RS-485 Half-duplex, Multi-drop Differential 32 31 4000 ft (1.22 km) 1 Mbps Termination Multi-drop Networks Differential Transmission