GE PLC ModBus 通讯手册

GFK-2220B 1 Modbus RTU Master Communications This document describes the operation of Modbus® RTU Master from the user interface point of view. Use this information as a supplement to the Serial Communications User’s Manual (GFK-0582). This document contains the following information: � Overview � Supported Products � Mode of Operation � COMMREQs � Time Constraints � Serial Connections � Standards � Multidrop Connections � Drivers and Receivers � Cable � Grounding � Connector Wiring � Four-Wire � Two-Wire � Point-to-Point � COMMREQ Function Block Format � COMMREQ Function Block Parameters � COMMREQ Function Block Output � COMMREQ Command Data Block � Modbus RTU Master Status Word Format � Queue Limitations for Modbus RTU Master COMMREQs � Local Data � Modbus RTU Master Diagnostic Status Words � Modbus RTU Master Commands � Initialize RTU Master Port: 65520 (FFF0) � Clear RTU Master Diagnostic Status Words: 08000 (1F40) � Read RTU Master Diagnostic Status Words: 08001 (1F41) � Send RTU Read/Force/Preset Query: 08002 (1F42) � Send RTU Diagnostic Query: 08003 (1F43) � Error Codes � Example Application ® Modbus is a registered trademark of Schneider Electric. 2 Modbus RTU Master Communications - December 2003 GFK-2220B Overview Modbus Serial Line protocol is an open standard for data communications between PLCs and related devices. The Modbus Serial Line standard provides for communication using either printable characters exclusively (Modbus ASCII), or binary data (Modbus RTU). This document describes Modbus RTU Master communications on GE Fanuc PLC CPUs. Supported Products Modbus RTU Master communications is currently available on the Series 90-30 IC693CPU363 and VersaMax® modular CPU models IC200CPU001, IC200CPU002, IC200CPU005 and IC200CPUE05. Mode of Operation A Modbus RTU master device (the client) sends query messages to one or more slave devices (the servers) on a serial network. Queries may contain data, requests for data or status, or commands. Each slave on the network has a unique device address. Any query may be addressed either to a specific slave device or to a special broadcast address. Queries addressed to the broadcast address are called broadcast queries. Queries that require a response may not be addressed to the broadcast address. A slave that receives a well-formed, non-broadcast query must send a response message to the master. The query/response transaction completes when the master receives a well- formed response. Slaves do not respond to broadcast queries. After sending a broadcast query, the master must wait a specified time before completing the transaction and sending the next query. Some broadcast queries contain commands that require the slaves to take specified actions. COMMREQs The application program running in the PLC CPU controls the timing and content of each query by sending a COMMREQ message. The COMMREQ must be addressed to the CPU serial port that is connected to the Modbus RTU serial network. COMMREQ data specifies the content of the query. When the query/response transaction completes, a COMMREQ status value indicates the success or failure of the transaction. See the COMMREQ Function Block Format section of this document and the sections that follow it for details. Time Constraints The Modbus over Serial Line Specification and Implementation guide V1.01 contains several important timing requirements. Serial Connections  Series 90 and VersaMax are trademarks of GE Fanuc Automation. 1 The current Modbus RTU specification is available online in the documents Modbus Application Protocol Specification V1.1 and Modbus over Serial Line Specification and Implementation guide V1.0 at http://www.modbus.org/. Follow the Modbus Standard Library link to find them. GFK-2220B 3 A Modbus RTU network has one master device and one or more (up to 247) slave devices. A serial network interconnects all these devices. If there is only one slave, a point-to- point connection is used. A multidrop connection is needed for two or more slaves. Standards Virtually all PLC serial communications ports (including all serial ports on GE Fanuc PLC CPUs) support one (or two or three in some cases) of three physical layer standards for asynchronous serial communications. The current revisions of all three may be purchased from the Telecommunications Industries Association at http://www.tiaonline.org/standards/. EIA/TIA-232-F: Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange (ANSI/TIA/EIA-232-F-1997) This standard is commonly referred to as “RS-232” or “RS-232C” because the definitive earlier revision was titled “RS-232-C”. RS-232 ports transmit and receive data and control signals on unbalanced circuits. That is, one Signal Common (or Signal Ground) wire serves as the return path for all the data and control circuits. RS-232 ports are suitable for point-to-point connections up to about 25 meters in length, but not for longer lines or multidrop connections. The specification recommends limiting the data rate to (in effect) 19,200 bits per second (bps) or less, but rates up 115,200 bps are frequently used with short cables (typically about 2 meters). EIA/TIA-422-B: Electrical Characteristics of Balanced Voltage Digital Interface Circuits (ANSI/TIA/EIA-422-B-94, revised 2000) This standard is usually called “RS-422” because the initial revision had that title. It specifies twisted-pair cabling and a balanced line driver and receiver for each circuit. RS- 442 supports higher data rates and longer distances than RS-232. A 100-ohm nominal impedance is recommended for twisted pair circuits in cables, and 100-ohm terminating resistors are recommended for the receiving end of each circuit. Some RS-422 ports support multidrop (multipoint) operation. However, this capability is not guaranteed by the standard. Use caution when attempting to use an RS-422 device on a multidrop network. EIA/TIA-485-A: Electrical Characteristics of Generators and Receivers for Use in Balanced Digital Multipoint Systems (ANSI/TIA/EIA-485-A-98) The original version of this standard was titled “RS-485”; it is frequently referred to by that name. This standard has effectively replaced RS-422 because it adds guaranteed multidrop (multipoint) capability. Line drivers in the data circuits are required to switch to a high- impedance state (“tristate” themselves) except when transmitting, and the control and status circuits are rarely connected through the cable in multidrop applications. Consequently, multiple data line drivers can be connected in parallel to each data circuit. The port firmware guarantees only one port at a time will attempt to transmit on each circuit. RS-485 uses 120-ohm cable and terminating resistors. Because transmitters are not always connected to the line, terminating resistors must be used at both ends of each circuit. Note that some RS-485 devices may require pull-up and pull-down resistors to polarize (bias) receive-data circuits to the mark state when all transmitters are in the high- impedance state. GE Fanuc RS-485 ports do not require pull-up and pull-down resistors. 4 Modbus RTU Master Communications - December 2003 GFK-2220B Multidrop Connections Four-Wire (4-Wire) In this network configuration, the Send Data (SD) pair of the RTU master device is connected to the Receive Data (RD) pairs of all the slaves, and the SD pairs of all the slaves are connected to the RD pair of the master. The slave devices must all use RS-485- compatible serial ports so that their transmitters are disabled except when transmitting. Although some RS-442 devices disable outputs when not transmitting, the RS-442 specification does not require it. The master may use either an RS-422 or RS-485 port because it is the only transmitter on that pair. Serial ports on all devices should be configured for Flow Control NONE. Both signal pairs must be terminated at both ends by appropriate resistors. Each pair is terminated at one end by the resistive terminator (RT pin) build into the end port. The other end of each pair is terminated by a 120 ohm, ¼ watt resistor in the connector shell. If the end device lacks a built-in terminator, a second 120-ohm, ¼ watt resistor must be wired across the RD signal pair inside the connector shell. Four-Wire Connection without Repeaters CTS(A’) RD(B’) RTS(B) SD(B) MODBUS RTU MASTER (END CONNECTOR) Notes: 1. Connectors on the cable ends have the Resistive Termination (RT) pin connected. All others have RT unconnected. 3. One connector is shown between the end devices. Additional connectors may be added up to a total of 31. 4. Attach all connectors to 15-pin RS-485 ports on GE Fanuc CPU modules. The Modbus RTU master device must be attached at the left-hand connector. All other devices are slaves. 5. Jumper connections between CTS and RTS are optional. No GE Fanuc 15-pin RS -485 ports that support Modbus RTU currently require them. 6. There are ground loops through the SHLD connections and frame ground connections of the master and slave devices. Large ground loop currents can cause data errors. See the accompanying text for a discussion of this issue. 7. Vary the connector wiring as needed to accommodate third-party RTU slave devices. SHLD N/C N/C N/C +5VDC RTS(A) SG CTS(B’) RT SD(A) RD(A’) 1 2 3 4 5 6 7 8 9 12 13 10 11 14 15 SHLD N/C N/C N/C +5VDC RTS(A) SG CTS(B’) RT RD(A’) RD(B’) SD(A) SD(B) RTS(B) CTS(A’) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 SHLD N/C N/C N/C +5VDC RTS(A) SG CTS(B’) RT RD(A’) RD(B’) SD(A) SD(B) RTS(B) CTS(A’) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 END CONNECTOR SLAVE CONNECTORS 120 120 2. Terminate the SD signal pair in both end connectors with a 120 ohm, ¼ watt resistor in the connector shell. Any high-quality shielded twisted-pair cable with two pairs is suitable for short cable runs (up to about 15 meters). Longer runs require a cable with a nominal impedance of 120 ohms. Use a cable designed for RS-485 transmission such as Belden2 9842 or equivalent. 2 Belden is a trademark of Belden Technologies Inc. GFK-2220B 5 Grounding and Ground Loops Proper grounding of the cable shield requires careful planning of the network and its power wiring. To avoid data errors from intermittent electrical noise, it is vital to ground the cable shield to the SHLD (shield) pin of every device on the network. Unfortunately, this introduces at least N-1 ground loops, where N is the number of devices on the network. Each ground loop path comprises the shield and drain wire on the cable segment between two devices and a ground return path. The return paths start at the frame ground point of one device, pass through its ground conductor to the common ground, and then pass through the ground conductor of the other device to its frame ground point. Ground loop currents must be kept within acceptable limits by careful grounding. Otherwise, common-mode noise induced on the data pair by the ground loop currents can cause data errors. When designing ground wiring, consider these requirements: 1. There must be one common ground point in the system with an extremely low impedance path to earth. 2. The conductor from the frame ground point of each device to the common ground must have extremely low impedance. 3. The recommended frame ground wire sizes, lengths and proper wiring practices must be observed in designing the connections between frame ground points and the common ground. 4. The data cable and ground wire routing must be physically isolated from other wiring that could couple noise onto the data cable or ground wiring. 5. If disconnecting the cable shield from the SHLD pin on any device reduces data errors, the network has a ground loop issue. Some older versions of GE Fanuc manuals recommend connecting cable shields at one end only to eliminate ground loop currents. This is no longer a recommended practice. When a shield is grounded at only one end, the network’s susceptibility to intermittent data errors from electromagnetic interference (EMI) is increased significantly. These errors may not be immediately apparent and often result in substantial post-installation costs for diagnosis and remediation. If data errors caused by ground loops cannot be avoided (for example, because the cable run is too long for all devices to use a common ground point), add one or more optically isolated RS-485 repeaters to the network. Partition the network into segments so that each segment has a common ground. Isolate the segments with repeaters such as the BLACK BOX model IC158A: http://www.blackbox.com/. See the repeater manufacturer’s data sheet for details. The figure on the previous page shows a typical 4-wire network without repeaters.  BLACK BOX is a registered trademark of Black Box Corporation. 6 Modbus RTU Master Communications - December 2003 GFK-2220B Four-Wire Connection Using Master RS-232 Port The master may also use an RS-232 port that is connected through an RS-232/RS-442 or RS-232/RS-485 converter. For example, the mini-converter provided in the GE Fanuc IC690ACC901 Cable and Miniconverter accessory may be used. This device is powered from power pins in the 15-pin RS-485 connectors on GE Fanuc PLCs. The power and ground pins should be connected to just one slave device as shown below. Here again, both signal pairs must be terminated at both ends by appropriate resistors. The transmitter in the mini-converter is always enabled and acts as a line terminator on the signal pair connected to its SD pins. The other end of the same pair is terminated by the resistive terminator (RT pin) build into the end port. The other signal pair is terminated by a 120-ohm, ¼ watt resistor in the connector shell at each end. If the end device lacks a built-in terminator, a second 120-ohm, ¼ watt resistor must be wired across the RD signal pair inside the connector shell. 120 END CONNECTOR 15-PIN SIDE OF MINICONVERTER FOR RS-232 MODBUS RTU MASTER Notes: 1. Attach the Modbus RTU master device through its RS-232 port and mini- converter at the left-hand connector. All other devices are RS-485 slaves. 2. Terminate the RD pair at the mini- converter with a 120 ohm, ¼ watt resistor in the connector shell. The mini- converter has an internal jumper between pins 9 and 10. 4. Connect the Resistive Termination (RT) pin only at the slave on the far end of the cable. All others have RT unconnected. 5. Jumper connections between CTS and RTS are optional. No GE Fanuc 15-pin RS-485 ports that support Modbus currently require them. 6. One connector is shown between the end devices. Additional connectors may be added up to a total of 8. 7. There are ground loops through the SHLD connections and frame ground connections of the master and slave devices. See the accompanying text for a discussion of this issue. SHLD N/C N/C N/C +5VDC RTS(A) SG CTS(B’) RT SD(A) SD(B) RD(A’) RD(B’) RTS(B) CTS(A’) 1 2 3 4 5 6 7 8 9 12 13 10 11 14 15 SHLD N/C N/C N/C +5VDC RTS(A) SG CTS(B’) RT RD(A’) RD(B’) SD(A) SD(B) RTS(B) CTS(A’) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 SHLD N/C N/C N/C +5VDC RTS(A) SG CTS(B’) RT RD(A’) RD(B’) SD(A) SD(B) RTS(B) CTS(A’) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 SLAVE CONNECTORS 120 3. Terminate the SD pair at the end of the cable farthest from the mini-converter with a 120 ohm, ¼ watt resistor in the connector shell. The number of slave ports that RS-442 and RS-485 masters can drive is somewhat less than the theoretical values in the two specifications. In practice, masters with RS-422 ports can drive six or seven slaves, while RS-485 masters can drive about 15 to 18 RS-485 slaves. The exact number will vary with the cable length and the locations of slaves on the cable. For additional slaves, add one or more RS-485 repeaters to the network (for example, BLACK BOX models IC155A and IC158A). See the repeater manufacturer’s data sheet for details. GFK-2220B 7 Two-Wire (2-Wire) Because only one device at a time can transmit data, Modbus RTU supports networks using just one data pair. Two-wire operation offers the advantage that cable cost is lower. Any device on a 2-wire network may be configured as the master. On a 2-wire network, the Send Data (SD) and Receive Data (RD) pairs of all devices are connected in parallel to a single pair of wires. Both ends of the pair must be terminated with 120-ohm resistors. All devices must be RS-485-compatible in order to disable their transmitters except when transmitting. All devices must disable their receivers while transmitting. Note that the GE Fanuc IC690ACC901 mini-converter does not meet these requirements. The signal pair is terminated at both ends by the resistive terminators (RT pin) build into the end ports. If either end device lacks a built-in terminator, a 120-ohm, ¼ watt resistor must be wired across the signal pair inside the connector shell. Any high-quality shielded twisted-pair cable is suitable for short cable runs (up to about 15 meters). Longer runs require a cable with a nominal impedance of 120 ohms. Use a cable designed for RS-485 transmission such as Belden 3105A or equivalent. Serial ports on all devices should be configured for Flow Control NONE. Typical Two-Wire Connection Notes: 1. Connectors on the cable ends have the Resistive Termination (RT) pin connected. All others have RT unconnected. 2. One connector is shown between the end devices. Additional connectors may be added up to a total of 31. 3. Attach all connectors to 15-pin RS-485 ports on GE Fanuc CPU modules. One Modbus RTU master device may be attached at any connector. All other devices are slaves. 4. Jumper connections between CTS and RTS are optional. No GE Fanuc 15-pin RS-485 ports that support Modbus RTU currently require them. 5. There are ground loops through the SHLD connections and frame ground connections of the master and slave devices. See the accompanying text for a discussion of this issue. 6. Vary the connector wiring as needed to SHLD N/C N/C N/C +5VDC RTS(A) SG CTS(B’) RT RD(A’) RD(B’) SD(A) SD(B) RTS(B) CTS(A’) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 SHLD N/C N/C N/C +5VDC RTS(A) SG CTS(B’) RT RD(A’) RD(B’) SD(A) SD(B) RTS(B) CTS(A’) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 SHLD N/C N/C N/C +5VDC RTS(A) SG CTS(B’) RT RD(A’) RD(B’) SD(A) SD(B) RTS(B) CTS(A’) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 END CONNECTOR END CONNECTOR accommodate third-party RTU slave devices. RS-485 repeaters can also be used on 2-wire networks. Point-to-Point When the network has only one slave device, a point-to-point connection between the master and slave is used. The cable connection may be either RS-232 or RS-422/485. Serial ports on both devices should be configured for Flow Control NONE. 8 Modbus RTU Master Communications - December 2003 GFK-2220B COMMREQ Function Block Format Modbus RTU Master communications use standard COMMREQ function blocks to o