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EIA-485 (RS-485)

Submitted by Visitor on 5 January, 2008 - 11:45

EIA-485 (TIA-485) Balanced (differential) interface defines a standard physical communications layer. EIA-485 is protocol independent and often called the RS485 standard (the term RS485 is out-dated). EIA-485 specifies bidirectional, half-duplex data transmission. Up to 32 transmitters and 32 receivers may be interconnected in any combination, including one driver and multiple receivers (multi-drop), or one receiver and multiple drivers. The EIA-485 communications standard, an upgrade of RS-422, supports the 32 devices per trunk without a repeater (multi-drop mode). The maximum cable length is 1200 meters (about 4,000 feet). Four-wire connections, which require an additional ground, require a 'master' node (e.g., a PC) that communicates to all others, called 'slaves,' which in turn can only communicate with the master. Capron Company rarely uses the 4-wire mode.
Termination
The RS-485 specification requires termination via an end-of-line resistor. For high baud rates and long cable runs, this is always true. In most equipment, with max speeds of 115kbit, the lack of the termination resistor may not be apparent in the operation of the system. Incorrect termination, such as the wrong resistor value or incorrect termination location, may cause lost or intermittent communication.
Extending the Network
By adding repeaters, you get longer distances - each repeater can drive another 1200 meters (3,900 feet) of cable - and 31 more RS-485 loads (driver/receiver pairs). Long networks are especially vulnerable to grounding and surge problems.
Number of Nodes (Controllers) per Trunk
The RS-485 specification requires support for a minimum of 32 devices for each trunk. This trunk loading specification is based on the Unit Load (UL) of each device being 12K ohms. The maximum devices on the net may be increased if the devices represent less then the UL, or have a "fractional" unit load. Some devices are manufactured to have a fractional UL as low as 1/4 or 1/8 the UL. A maximum of 256 device could exist on the bus when each is at 1/8 the UL (96k ohms). Cylon's unitary (UCU) controller fieldbus network, which communicates via an EIA-485 trunk, can accommodate up to 127 controllers on a single segment when operating as BACnet controllers, but a lower number is often applied to accommodate network speed and loading.