Designing and Installing an RS485 Serial Network

[Music] welcome to another edition of prosoft Technology video training serial networks are the simplest way to send ones and zeros over a wire a robust and efficient serial network is the result of good planning and proper installation in this video we will cover the design and installation considerations of setting up an rs485 serial Network serial Communications come in a variety of different physical standards rs232 and rs45 being the most common in industrial environments unlike rs232 which is a point-to-point short distance Network rs45 can support up to 32 devices in a half duplex multi-drop configuration it can also support Communications up to 4,000 ft or 1200 M and support B rates from 110 to 115.2k the communication range can be extended even further with an rs45 repeater as a result rs45 has become the basis of the most commonly used Industrial Automation communication protocols like modbus rtu optimux profibus DP and dh485 the wiring for rs45 comes in two flavors two wire which actually uses three wires and four WI wire which actually uses five in each case the extra wire is the signal common and we'll talk more about that later in a two wire Network the transmitter and receiver of each device are connected to a pair of wires positive and negative four wire networks have one master port with the transmitter connected to each of the slave device receivers on one pair of wires the slave transmitters are all connected to the master receiver on a second pair of wires the two wire systems are more popular but which flavor you use will depend on the devices and protocol that you want to use in both types of networks each device or node is addressable and can be communicated with independently the ideal layout of an rs45 network is a daisy chain architecture with the master device located at one extreme end this tends to minimize the possibility of signal Collision star T drop or ring type topologies are strongly discouraged as they can cause serious communication problems in industrial environments the problem of electrical noise being picked up on the transmission line can constitute a serious challenge to wired communication for best results use twisted pair cables for the signal Pair by twisting back and forth the noise current generated by external interference affects the signal levels in both wires in roughly the same way effectively cancelling itself out and leaving the voltage difference on each line largely unaffected this is what gives rs45 its impressive range on a network with proper topology you only need termination resistors at the two extreme ends of your network in order to prevent signal Reflection from an open circuit some devices have a resistor built in if you use such a device and the resistor is engaged it should only be placed at the extreme ends if your devices do not have built-in termination note that the value of each resistor should be equal to the characteristic impedance of the wiring typically 120 ohms for Twisted pairs another fundamental problem of wired electrical communication is that the Earth has different potentials at different locations the ground potential on Industrial sites can be vastly different ground potential differences of more than 12 volts could possibly damage an rs45 port to solve this problem or at least fight against it rs45 networks utilize a signal common wire and a shield ground the signal common wire runs along with the signal wires between each node on a network it provides a common reference potential level for the rs45 nodes which otherwise carry floating voltages the signal common ensures that each device will have the same Zer volt reference and will not inadvertently exceed the voltage limits of the hardware while transmitting data because of potential level differences in the transmitter and receiver locations it's also typical for communication lines to have a shield a thin metallic jacket encasing the signal wires that is conductive and grounded The Shield ground is a notional infinite charge sync that allows the shield to act as a faraday cage keeping out external electromagnetic interference from disturbing the signals in the communication wires even though twisted pair cables largely negate the effects of external interference it's still good practice to use shielded cables when wiring up your network use the common signal reference cable to time the ground potential of each port to the same ground you should also ground The Shield at only one point in the network if the shield is grounded at several points you may get ground Loops current flowing through the shield and this will induce noise on your signal wires speaking of inducing noise on your signal wires when setting up cabling for your network be sure to run your signal lines in a different conduit than your power lines there are limits to how much electrical interference a shielded twisted pair wire can negate keep in mind that not all manufacturers label their wires the same if your prosoft master device constantly transmits but receives no reply try swapping the two signal wires there's no real standardization for the labeling of rs45 signal wires so this is often the source of communication problems if you have an rs45 four wire network but want to use a two-wire device you can simply tie the two positive signals together and the two negative signals together at each device and you'll have a two-wire device communicating over four wires all types of industrial cables are not created equal be sure to use the correct cable for Optimal rs45 Performance this is a 120 ohm nominal impedance cable such as the Beldon 3106 6A industrial cable these are some of the basic considerations and tips that you should keep in mind for an rs45 network if you would like more information or technical assistance don't hesitate to contact [Music] us

Channel: ProSoft Technology

Views: 192,154

Keywords: prosoft, technology

Id: VhzjYecxiSg

Length: 7min 23sec (443 seconds)

Published: Thu May 19 2016