Clear And Unbiased Facts About RS485 Standard (With out All the Hype)

RS-485 has a single pair of wires. The signals appear differentially on two wires of each pair. RS-485 figure 2 shows a generator (driver) with two interface connection points labeled "A" and "B". RS-485 simply defines the interface connection points as "A" and "B" and shows the voltage relationship between "A" and "B" for the binary states of the two wires, not the binary state of the input to the driver. The result of this looks like two 120 Ω resistors in parallel (60 Ω) to either end or to any point on the cable. Half of the conductors in a 50 way cable are ground return wires. The driver must output between 5V and 15V in magnitude into a load of 3kΩ to 7kΩ. The driver must not be able to output more than 100mA when shorted to any other conductor in the cable, must not be able to output more than 25V and must be able to handle an open circuit, or a short to any other conductor in the cable.

The driver must not exceed 10V differential, or 6V common mode (outputs relative to circuit common). 10V difference. Beware of ground loops when using the shield as the third wire. The exact voltage of high and low are not usually specified, rather a voltage difference between the two outputs is specified. Electrical Signaling Techniques is an important parameter that determines the noise immunity of the two protocols. Hysteresis is typically incorporated in the receiver to improve noise margin, but is not required. To add a little extra noise margin, RS485 standard use 680 Ω resistors and you have one of the more common biasing networks used on a RS-485 network. We recognize that environmental noise at some project sites can lead to issues that may require additional hardware and labor. If applications require connecting multiple systems, higher data rates, more noise immunity, and the distance is long, then RS-485 is a more suitable candidate.

When the driver is transmitting a 1 (from the UART for example), then the voltage on the wire must be less than -3V. This third example would be a good place to use an isolating RS-485 transceiver. Since the laptop has no connection to earth ground, there will normally be little current through this third wire. The annex of RS-485 states that "When employed, the shield shall be connected only to frame ground at either or both ends depending on the specific application." Shields usually have a lot more copper (and/or aluminum) than a single wire and can therefore carry a lot more current. The current could cause damage to the wire, or the RS-485 port. The desktop is connected to earth ground and the RS-485 port is referenced to the earth ground. The RS-485 circuit common is connected to earth ground in both PCs. RS-485 voltages are referenced differentially from one wire to the other, but are also referenced to circuit common. Serial Communications Example Program Features of WSC4C include: - Free technical support and updates for one year. This example of biasing would be a bad thing on an equivalent RS-485 network.

Digital logic usually uses the same convention (but not the same voltage) as the lamp example. But a lot of RS-485 hardware uses only 1 pair of wires (half-duplex). RS485 uses the same differential signaling scheme as RS422, and hence has the same superior signal-to-noise characteristics and range described above. A thousand feet of 24 AWG wire is in the 26Ω range. RS-232 typically has a transmit wire, receive wire and signal common wire. The driver must not exceed 6V differential, or 6V common mode. This means the specification has a differential voltage requirement, and a common mode voltage requirement. This means there will be times when no driver is connected to the wires. With no driver connected, the differential voltage on the wires will depend on the termination resistor and biasing. Adding the biasing resistor to every device on the network can cause termination problems. A termination resistor can increase the signal's quality (reduce ringing), but it will also reduce the signal's amplitude. When a dry contact closure is detected, the unit can perform a wide variety of functions such as recall a scene or perform a control room join.