- HOW TO TEST RS485 COMMUNICATION INSTALL
- HOW TO TEST RS485 COMMUNICATION FULL
- HOW TO TEST RS485 COMMUNICATION PLUS
Our advice: For longer networks with lots of devices, choose 38k400 baud over 76k800 baud and optimize using COV, separate networks and by setting the Max Master to a lower number. However, the higher the baud rate the more sensitive the cable is to the quality of installation issues like how much twisted pair is unwound at each termination start to become very very important. For example, at 115k baud your cable should not be much longer than 2500 feet. Above that, you need to do a little math and reduce the length. Practically speaking you can go up to 4000 feet at baud rates up to 76800 baud.
HOW TO TEST RS485 COMMUNICATION INSTALL
Thus to see how many devices you can install you simply get the data sheets and add the loads. Today you get half and quarter load devices.
HOW TO TEST RS485 COMMUNICATION FULL
Since the standard was developed most RS485 chips present less than the full unit load originally specified. The RS485 standard is based on 32 devices. The simple answer is to use a repeater but in practice one isn't always necessary. How do you put more than 32 devices on a single RS485 trunk?
HOW TO TEST RS485 COMMUNICATION PLUS
Multi-Conductor EIA Industrial RS-485 PLTC/CM22 AWG stranded (70) tinned copper conductors, Datalene insulation, twisted pairs, overall Beldfoil shield (100% coverage) plus a tinned copper braid (90% coverage), drain wire, UV resistant PVC jacket.Ħ. Use one pair for Tx, Rx and a conductor from another pair for the ground reference signal. Choose low capacitance cables.Ĭan you use Cat5 cable? Yes. Wrong in the sense to determine the value of terminating resistors now requires measurements and calculations. If they quote an impedance per meter/foot you have chosen the wrong kind. If they quote a number such a 100Ohms you have a good cable. A clue to knowing if you selected one is to look at the cables Nominal Impedance. Some cables are designed so that the impedance is relatively independent of distance. Cable selection does make a difference.Īll cables offer impedance (resistance). Another big culprit are Variable Frequency Drives.ĥ. People are often surprised to find that the worst source of induced noise are switching DC loads. It seems obvious not to wind your cable around other cables or sources of electricity/magnetism. The twisted pair used for Tx and Rx is more effective at noise cancellation than the shield.Ĥ. Always connect the ground reference conductor first if you are connecting a device that is powered up or you are connecting your laptop to an operating network.Īlways choose devices that have optical isolation this almost always will protect the RS485 transmitter/receivers.ģ. You can (if you must) use a shield drain wire as the 3rd conductor (ground reference conductor)Ģ. The voltage level on the Tx/Rx conductors are measured relative to this voltage level. Remember this statement: The so-called Ground Terminal on an RS485 interface is not a connection to ground. The more power sources used to power devices, the greater the physical separation of devices, the less well-grounded devices, and power sources are the greater the risk. For a more detailed discussion read this article. You risk blowing 485 ports, you risk unstable operation (works sometimes and doesn't work other times) and finally, you risk re-installation. You take a huge risk by not installing the 3rd conductor. What he means is that RS485 is often non-trivial and getting a network working can rely more on experience and experimentation.ġ. It is his reference to Art that makes RS485 bad. Search the Internet on RS485 you will find Bob Perrins's article called THE ART AND SCIENCE OF RS-485. Need a BACnet Integration Solution? Click here for more informationīACnet MSTP uses RS485 (also known as EIA485) as it physical layer.