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Some manufacturers have avoided this confusion by changing the name of the output pins labels to X and Y. There are two things to be noted about this so called "pin mislabeling". Finally there is confusion from the IC manufacturers. ProfiBus defines an RS-485 type bus using repeaters. The difference between an adapter and a converter generally is that an adapter usually only adapts the physical interface of a device to match a different type of physical interface of a different device whereas a converter converts the actual data stream between two devices. This page provides two examples of RS485 fault protection. Cabling standards are listed lower down the page. Three different diode configurations and part numbers are shown on the DIP Package Diode Array page. Three included mechanical relays can be used for process control like interlocking valves or pumps. Each 24-pin grouping is subdivided into three ports labeled A, B and C. The 8-pins within the different ports are defined as A0 through A7, B0 through B7 and C0 through C7. RS232 ports on the PC are assigned to a single device.

Single board solutions may be hooked up as shown, but chassis to chassis systems require a common ground connection to run between the driver and receiver. In a typical applications several address able devices may be linked to a single controlled (PC), and in this way a single line may be used for communication. Repeaters may be used to extend the net to any number of devices. A number of devices are being produced which represent 1/4 or 1/8 the unit load. Currently RS485 drivers are produced that can achieve a bit rate of 35 mbps. Pull-up/Pull-down resistors (Idle-line failsafe) at one end of the 485 bus may be used to bring the line voltage to a steady state (200mV) value at the end of a transmission; when all drivers are in the passive state. EIA-485 devices may be used in 422 circuits, but EIA-422 may not be used in 485 circuits (because of the lack of an Enable line). However, the circuits should be grounded. Circuit grounding is not shown in either of the RS422 or RS485 circuits. Cabling is not specified in RS422 or RS485, but a rule of thumb is listed above for 24 AWG twisted pair copper wire with 16pF/ft. Refer to 24AWG Attenuation vs Frequency Chart.

The "pin mislabeling" discussed above is a good starting point. To achieve good long distance noise immunity, 2 wires are used to carry each signal, configured as a twisted pair of cables. Each of the 24 SMINI input lines feature optional input line filtering for maximizing immunity to electrical noise frequently encountered with pulse-type command control systems such as DCC and Railcommand. The TXD pair and RXD pair are used to carry the data whilst the RTS pair and CTS pair lines are used for handshaking. Often only one twisted pair cable is used as the TXD and RXD lines are tied together; this is known as half duplex mode. Some CPUs/UARTs (e.g., Atmel AT91 or 16C950 UART) contain a built-in half-duplex mode capable of automatically controlling line direction by toggling RTS or DTR signals. EIA-485 specifies bidirectional, half-duplex data transmission. For half-duplex transceivers, both the driver and receiver utilize shared ESD protection. 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. A number of specifications use EIA-485 as the electrical standard, including SCSI-2, SCSI-3, BitBus, and Profibus. EIA-485 requires a 120 Ohm line impedance (normally Shielded Twisted pair 'STP'), 120W (10%, 1/2 watt) terminations at both ends of the line (at the Receivers).

Only one end of the bus requires the resistor network. One RS-485 network can be used for the master to talk to all of the slaves on the network, and a different RS-485 network can be used for all of the slaves to talk back to the master. Stub length, termination, and biasing resistors can have a significant impact on the performance of the network. The resistors are normally required once the system has reached it's quiescent state, when no drivers are driving the bus. Normally required once the system has reached it's quiescent state, when no drivers are driving the bus. 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. The drivers and receivers should be connected directly to circuit ground. The number of loads, the termination resistance 60 ohms (120 ohms per side), and the required (turn-on) current of the receivers. 60 ohms (120 ohms per side), and the required (turn-on) current of the receivers.

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