Our products are at the South Pole, the bottom of the sea, the edge of space, and in between. Many of our products were developed in response to a customer’s specific need. It is also used in Digital Command Control (DCC) for model railways. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. RS-485, also known as TIA-485(-A) or EIA-485, is a standard, originally introduced in 1983, defining the electrical characteristics of drivers and receivers for use in serial communications systems. These manufacturers all agree on the meaning of the standard, and their practice is in widespread use. Although many applications use RS-485 signal levels, the speed, format, and protocol of the data transmission are not specified by RS-485. The diagram below shows potentials of the A (blue) and B (red) pins of an RS-485 line during transmission of one byte (0xD3, least significant bit first) of data using an asynchronous start-stop method. Figure 1 shows an example of a correct Bus connection. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. The cabling of the industrial communication systems (Modbus RS485) is different in some ways from the cabling used for power cabling and the electrician may experience some difficulties if he is not an expert in Modbus communication networks.

The adapter includes a power supply, 6' serial cable and User's Manual. Industrial strength: Wide power supply, temperature and humidity tolerance. These characteristics include: definition of a unit load, voltage ranges, open-circuit voltages, thresholds, and transient tolerance. However this contradicts the facts that an idle mark state is a logical one and the termination polarization puts B at a higher voltage in Profibus guidelines. If A is negative with respect to B, the state is binary 1. The reversed polarity (A positive with respect to B) is binary 0. The standard does not assign any logic function to the two states. RS-485 standard conformant drivers provide a differential output of a minimum 1.5 V across a 54-Ω load, whereas standard conformant receivers detect a differential input down to 200 mV. The EIA once labeled all its standards with the prefix "RS" (Recommended Standard), but the EIA-TIA officially replaced "RS" with "EIA/TIA" to help identify the origin of its standards.

The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, i.e. a line or bus, not a star, ring, or multiply connected network. 12 V, i.e. ±7 V on top of the 0-5 V signal range. In order to avoid signal reflections, a 120 Ohm termination resistance must be fitted on each end of the main cable. The RS485 system used for Modbus communication provides a main cable (Bus or backbone), to which all the devices have to be connected with branches (also known as stubs) that are as short as possible. Henceforth, we shall consider Slave devices to be measuring instruments with serial communication, even if the cabling is similar for all Modbus devices. Texas Instruments (Technical report). RS-422 and RS-485 Standards Overview and System Configurations, Application Report (pdf). RS-485 and RS-422 can interoperate with certain restrictions. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. In theatre and performance venues, rs485 cable RS-485 networks are used to control lighting and other systems using the DMX512 protocol.

By using repeaters very large RS-485 networks can be formed. RS-485, like RS-422, can be made full-duplex by using four wires. 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). The maximum number of devices that can be connected to a main cable is 32, including the Master. As a rule of thumb, the speed in bit/s multiplied by the length in metres should not exceed 108. Thus a 50-meter cable should not signal faster than 2 Mbit/s. RE Smith devices will not limit your communication speed. It does not specify or recommend any communications protocol; Other standards define the protocols for communication over an RS-485 link. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. These are used in programmable logic controllers and on factory floors. For an off, mark or logic 1 state, the driver's A terminal is negative relative to the B terminal. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals.