Combined with the right cables, they are going to guarantee the creation of stable and functional RS485 networks. 24 AWG solid wires, which are typically used in Cat 5 cables, can break easily when connected to a screw terminal block, which does not provide any strain relief. The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). Differential signals can help nullify the effects of ground shifts and induced noise signals that can appear as common mode voltages on a network. This works well and prevents the existence of ground loops, a common source of communication problems. SC, G or reference, the common signal reference ground used by the receiver to measure the A and B voltages. Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. But these are signal wires and the network always requires a return path.

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. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. Check out also our article about BIAS and TERMINATION resistors. A simple method is often a status check. Now in the loop, we check if there is anything available on the serial port and then read the data and since the incoming data is an integer, we parse it and display on the connected LCD. You can implement the slave select lines by configuring Port A pins as outputs. The PDQ Board, however, does not implement hardware handshaking. This allows RS-485 to implement linear bus topologies using only two wires. This detects the presence of more than one master on the SPI bus. Multiple receivers may be connected to such a network in a linear, multidrop bus. 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.

The cable recommended in this type of connection is the Belden 9842, rs485 cable which has 2 twisted pairs. This works and is sometimes also recommended by many vendors. The shield works as a "drain" for any noise that could be picked up by the RS485 network and "drain" it to ground. End of line termination resistor (120 ohm 1/4 watt) to be installed at both ends of network. RC termination - In some applications requiring low power you may not want to load the line with 120 Ω resistors at each end. TIA-485 and V.28/V.24 communication interface with low power requirements. In the simplest scheme, all RS485 transceivers come up in receive mode when the interface is initialized, and each transceiver node has a unique address known to it and the master. The master initiates a communications request to a "slave node" by addressing that unit. A single master can broadcast commands to all the slaves, and can direct commands to an individual slave using its unique address. After the slave transmission is complete, the slave puts itself back into receive mode so that the master can transmit additional commands. Setting the MSTR bit initializes the QScreen as a master, and clearing the MSTR bit initializes it as a slave.

To ensure that no two devices drive the network at the same time, it is necessary that each slave device be able to disable it’s own RS-485 data transmitter. RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422. RS485 uses the same differential signaling scheme as RS422, and hence has the same superior signal-to-noise characteristics and range described above. It operates on a differential signaling method of measurement rather than voltage measurement wrt GND pin. Electrical signaling is balanced, and multipoint systems are supported. Also, when some devices on the RS485 are NOT opto-isolated, the third wire becomes essential to avoid the units being damaged. So, how do I wire my RS485 devices? The ideal RS485 network requires a dedicated wire for ground. The RS485 network is wrongly referred to as a "2 wires plus shield" network. It is NOT necessary to introduce long delays in a network to avoid "data collisions." Because delays are NOT required, networks can be constructed, that will utilize the data communications bandwidth with up to 100% through put. If a star configuration is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans.