Understanding and Implementing a Functional RS-485 Network

Transcription

1 Understanding and Implementing a Functional RS-485 Network SCOPE This article is intended to help clarify the details and correct application of a RS-485 network using Detcon equipment. It is also to help in understanding what RS-485 is, how to make it operate properly, and demystify some of the myths that seem to make it an undesirable and hard to install communications link. It is actually just the opposite. Also included are some basic trouble-shooting tips and cable type recommendations. By no means is this a full and complete document on the subject, but it should address some of the more common questions and problems. RS-485 : THE SPECIFICATION RS-485 is a generic electrical specification only. It does not specify connector types, protocols, or cable type. It is the responsibility of the installer to provide a correct physical installation for the RS-485 network to function properly. RS-485 is very inexpensive and inherently rugged, allowing multiple devices to communicate over a single twisted pair of wires. Detcon has chosen to implement the Modbus protocol using the RS-485 electrical specification. RS-485 is a two way half-duplex data bus made up of a transceiver (transmitter and receiver). In it s simplest form, one transceiver is located on each end of a twisted pair cable of up to 4000 feet maximum. Data can be sent in either direction, but can only be sent in one direction at the same time. It is like having a one-lane highway that is used for travel in both directions. The vehicle going east must wait while the vehicle going west uses the highway, and vice-versa. In this case the protocol that is used becomes the traffic controller. The specification allows for the connection of up to 32 unit loads (UL s) to the twisted pair. An older transceiver chip would have represented 1 UL. The current chip that Detcon uses on its equipment represent a 0.25 UL therefore theoretically allowing 128 devices to be connected to the twisted pair (data bus). Connection of more than 32 UL s to the bus will limit the drivers and cause attenuation of the signal preventing the system from operating properly. SYSTEM WIRING Since RS-485 allows multiple devices to be connected to the bus, wiring is not as straight forward as it is in a bus with only 2 devices. The best wiring scheme is the daisy-chain connection from device1 to device 2 to device 3 to device n. All devices that tap into the bus should not be at the end of long stubs, branches, or t-taps. A true daisy-chain has direct wiring between devices with not even minor branches. Detcon equipment contains four wiring terminals for 485 connection. Two for A(+) and two for B(-). All connections should me made directly to these. Figure 1 shows an electrical representation of a simple RS-485 bus with 4 transceiver chips. Figure 1. Page 1 of 7

2 The use of a daisy-chain connection will perform the best in all applications given the bus is properly terminated. The standard specifies the transceiver characteristics only, not the interconnection of the transceivers themselves. Depending on the environment, distance and other factors you can connect transceivers using flat cable, twisted pair cable, even PCB traces. However for the most reliable data transfer in industrial environments shielded twisted pair is required. 18 to 28 AWG has been used, but 24 AWG is highly recommended. The characteristic impedance of the cable must be between 100 to 120 ohms. Twisted pair is used because if the cable does run near a noise source both conductors will pick up the same amount of noise, therefore effectively canceling it out. Some of the biggest problems with an RS- 485 bus is the use of an incorrect wiring scheme mixed with improper line termination. Most installations are not used to having to run wires in a daisy-chain fashion, instead wire runs tend to originate from a central point and run to different devices in a star-like pattern. This is the absolute worst way to wire an RS-485 bus. The chances of worry free operation over the life of the equipment decreases dramatically the further you stray from a straight daisy-chain wiring scheme. The bus needs to be terminated at each end of the cable run to operate ideally. With other wiring schemes you introduce multiple cable ends making it impossible to balance the lines. Figure 2. shows some examples of good vs. bad wiring schemes. Wiring as in Figure 2 (A,B & C) would more than likely require no special fixes to allow reliable data flow. Figure 2 (D,E & F) could possibly be made to operate, but would require many repeaters and man-hours to do so. GROUNDING Figure 2. Another problem that can occur with RS-485 is incorrect grounding. Neither one of the two conductors in the cable is a GND. Both of the conductors are supplying a current to maintain a voltage level relative to an external reference. You must either supply a third conductor as the reference, or establish the reference through earth ground. RS-485 is specificed to be able to work normally with a +/-7V ground potential difference and survive +/-25V surges. In most applications our equipment is powered off of our own DC power supplies. This is good as long as the supplies are located in Page 2 of 7

3 the same physical location, and their DC commons are tied together, and then tied to earth ground. Problems can occur when you have part of the data bus powered off of one supply and the second part of the bus powered off of a power supply located somewhere else in the plant. In this case you are relying on earth ground to be the reference between the two sections of the bus. If noise gets induced onto the earth ground of one power supply and not the other, you may wind up with data errors. This is even more likely to occur when the distance between the ground references is long. A way around this is to install an isolated repeater into the data bus to isolate the grounds from each other, enabling the bus to use only one of the two references. USING REPEATERS Repeaters can play many rolls in the implementation of an RS-485 data bus. They can extend distance if you need to go further than 4000 feet, allow for the addition of more devices to then bus, increase signal strength and integrity, solve grounding problems and solve some of the problems that will occur when an incorrect wiring scheme is implemented. A repeater in it s basic form consists of two transceivers working together. One transceiver is connected to the main data bus and the other transceiver connects to the remainder of the devices as it creates a new and separate data bus. Figure 3 shows a data bus that implements a not so desirable branched wiring scheme. Depending on the length of the taps of slave 1 and slave 2, they may or may not communicate properly back to the master. The long lengths create 4 ends to the cable instead of just 2, which can not be balanced with the 2 terminating resistors on the extreme ends. Without a balanced bus the long taps of slave1 & 2 will introduce reflections to the signal that can lead to problematic operation in the field. These problems can be very hard to diagnose, much less isolate and fix. Figure 3. The addition of 2 repeaters (Figure 4.) can solve this problem. The repeaters look like short stubs to the main bus, and at the same time they create 2 new busses that have all the same characteristics as the main bus. Each new bus could have up to 32 UL s connected to it. Just don t forget the termination resistors to balance the new data bus. Figure 4. Page 3 of 7

4 Be sure to check the equipment you are installing. Some manufacturers include a built-in terminating resistor that can be selected with a jumper or dip switch. Detcon s Model 880 has such a jumper located on the FA-isonet card. The jumper is labeled 120 ohm Term. Detcon s 600 series gas sensors and DA1 modules do not include this resistor, and therefore, if one of these devices is the last on the RS-485 bus then a discrete 120 ohm resistor should be placed across the A and B terminals on the connector board. PROTOCOL Now that we have been through the communications medium, lets look at the language and mannerisms (Protocol) that are used to communicate between many devices. The same protocol could be used on an RS-232 port or an Ethernet port. The protocol does not care about the electrical side of things, it is simply a set of rules and specific details of how we will communicate. Detcon has chosen to implement Modbus RTU as its protocol of choice. Mainly because it is very widely used as an industrial protocol by many other companies and there are no licensing fees to use the protocol. As stated earlier 2-wire RS-485 allows communication by only one device at any given point in time. Therefore most protocols are implemented with one master device, and many slaves (unit loads). The master controlls all communications. A easy way to understand this is to think of being in school, where you can only speak when you are spoken to. The teacher would be the master, and the students the slaves. No one is ever talking at the same time. The master first turns on the transmitter section of its transceiver. The master then transmits a data stream. The data stream contains an address (485 ID) which corresponds to one of the slave devices, along with the instruction for that device, followed by a couple of error checking bytes. When complete, the master turns off its transmitter and waits for a response. Once the slave in question recognizes the command from the master it performs the instruction and prepares to answer the master. First it turns on its transmitter, then it sends a data stream. The data stream contains its own address (485 ID) along with the data that was asked for by the master, followed by a couple of error checking bytes. The slave then turns off its transmitter and all is quiet until the master requests additional data from one of the slaves. The Master transmits this data stream (HEX) C5 C8 01 is the slave address 03 means the master wants to get some information from the slaves registers is the starting address of the data is the number of registers it wants C5 C8 are the error checking bytes (CRC) The slave transmits this back to the master F3 6E 01 is the slave address 03 indicates the requseted data will fillow 08 means 8 bytes of register data will follow (it takes 2 bytes to make a register) data F3 6E are the error checking bytes (CRC) Page 4 of 7

5 DEVICE HOOKUP Installation should begin by deciding where devices will be located and how the connections between devices and the master will be established. The ideal scenario would look like the drawing shown below. Here we have an 880 Controller connected to 11 slave devices using a daisy-chain wiring scheme. This would require 2 different twisted pair cables, one for power and the other for our RS-485 data bus. The connections would be point-topoint starting at the master and going to the last slave. The RS-485 data cable should come into the sensor enclosure and be connected to the A & B terminals. The next segment of cable should also connect to the A & B terminals and leave the enclosure headed for the next device. The shields must now be tied together inside the enclosure and not allowed to short to any other wires or surfaces. The shield should be connected only inside the 880 cabinet to the shield terminal. The label in the drawing shows where the termination resistors would be installed. Although this is ideal, reality is never ideal and wiring may already exist or the wiring can not be run this way for some reason. Below shows a more realistic wiring situation that may occur. In this case it is impossible to balance the data bus because there is no Control Room 0a distinct beginning or end to the cable run. The best way to make this type of installation successful is to install repeaters in a few key areas to eliminate the t-taps or stubs which can cause problems with the signal quality. The location and number of stubs will dictate where repeaters need to be installed. The next drawing shows 4 repeaters installed to eliminate the stubs. Notice that there are now 5 different data buses that make up our communications network. 07 0b Control Room repeater #2 Control Room a 0b 09 repeater # a 0b repeater #3 repeater #4 08 Page 5 of

6 The first one consists of the master located in the control room, device 09, repeater #1, device 01, device 02, repeater #2, device 03, and device 05. Notice the termination resistors at the beginning and end of this bus section. The second bus starts at repeater #2. It consists of the repeater and device 04. Since this is a new bus it has terminating resistors at each end. The third bus starts at repeater #1. It includes device 08, repeater #3, device 07, repeater #4, and device 06. It also has its own resistors. The fourth bus starts at repeater #3 and consists of the repeater, device 0A, and the terminating resistors. And the fifth bus starts at repeater #4 and consists of the repeater, device 0B, and termination resistors. This configuration isolates all of the t-tap stubs and should function properly if wire type and distances are observed. The following chart shows an approximation of wire length vs. data speed. Detcon operates its equipment at 19,200bps and lower. 100 mbps 10 mbps 1 mbps 100 kbps 10 kbps 1 kbps 0 ft 10 ft 100 ft 1000 ft 5000 ft ft TROUBLESHOOTING Trouble-shooting RS-485 with multiple devices attached to the data bus can be an aggravating task. Sometimes one device will take down the entire bus, but if brought back to the shop, the same device might work just fine. Most of the time the failure is a damaged transceiver chip, which was subjected to an over-voltage or a surge. The first step in finding the bad device or devices is to start at the master. Disconnect the data wires (A & B). Connect a working device directly to the master and make sure that it is functional. If it is not, replace the master s transceiver chip or board. In the case of an 880 controller, this would be the FA-Isonet board. Once this is done stop the master from communicating, but do not power it down. On the model 880, this can be done by unplugging the phone jack connection to the FA-Isonet card. Now unplug all of the slave devices from the bus. When using 600 series transmitters or DA-1 s this is done by simply unplugging the transmitter from the connector board. Reconnect the data bus wires (A&B) to the master. Using a meter measure the voltage across the A and B wires and write this reading down. Go to the first slave device plug it in. Take another measurement and write it down. Unplug the first device and proceed to the next slave recording the meter readings as you go. The first measurement you took should have been arround 400 to 500 millivolts. If this is not the case, be sure you have unplugged all devices from the data bus. If you still don t measure somewhere arround 400 to 500 millivolts there may be a problem in the wiring that must be corrected. Now, if any of the other readings you took are above or below the first reading by more than 50 Page 6 of 7

7 millivolts, that units transceiver chip should be replaced. You should wind up with results similar to the ones shown here. As you can see slave device 03 and 0B need to be changed. Slave master MV Difference ID ID ID ID ID ID ID ID ID ID 0A ID 0B Also measuring at the master device between dc common and A, or dc common and B should give you a reading of around 2.5vdc. Remember this is with the power on, but no communications taking place. Another method of troubleshooting which is not as exacting is to enable the communications, but remove all the slaves. Proceed to plug 1 slave in and see if communications are still good. If so plug in the next slave and check again. Repeat this with each slave until you either have all of the slaves plugged in and everything is fine, or a single slave will take down the data bus. This is not the best way to troubleshoot an RS-485 data bus, but if you have no other means, it is a good start. Here are some recommended cables and their specifications from Belden. These were designed with RS-485 in mind, and should be used. Belden# pairs AWG Impedance ohm ohm ohm The following companies make RS-485 repeaters that we have used with good results. www,rs485,com Models IRSFC24FB, DIFAR44 www,robustdc,com Model RDC485IR3 Too many times have we seen problematic systems where the root cause of communications problems was incorrect wire type and wiring scheme. It is easier to avoid these problems before they actually are problems by just following a few simple guidelines. Have a good wiring scheme, use a recommended cable, properly connect your data lines and shields, don t forget about those terminating resistors and have some isolated repeaters handy in case you run into some grounding problems. References: Robust Data Comm AN-003, AN-005, AN-006, AN-021 Circuit Cellar The Art and Science of RS-485 Page 7 of 7