Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 350
Modbus
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Section 8. Operation 'Object group 30, variation 2 is used to return analog data when the CR800 'is polled. Flag is set to an empty 8 bit number(all zeros), DNPEvent is a 'reserved parameter and is currently always set to zero. Number of events is 'only used for event data. DNPVariable(IArray,4,32,2,3,&B00000000,0,10) DNPVariable(BArray,2,1,1,0,&B00000000,0,0) DNPVariable(BArray,2,2,1,1,&B00000000,0,1) Scan(1,Sec,1,0) 'Wind Speed & Direction Sensor measurements WS_ms and WindDir: PulseCount(WindSpd,1,1,1,3000,2,0) IArray(1) = WindSpd * 100 BrHalf(WindDir,1,mV2500,3,1,1,2500,True,0,_60Hz,355,0) If WindDir>=360 Then WindDir=0 IArray(2) = WindDir * 100 'Default Datalogger Battery Voltage measurement Batt_Volt: Battery(Batt_Volt) IArray(3) = Batt_Volt * 100 'Wiring Panel Temperature measurement PTemp_C: PanelTemp(PTemp_C,_60Hz) IArray(1) =PTemp_C PortGet(Barray(1),5) 'Update DNP arrays and send unsolicited requests to DNP Master address 3 DNPUpdate(2,3) NextScan EndProg 8.6.2 Modbus 8.6.2.1 Overview Modbus is a widely used SCADA communication protocol that facilitates exchange of information and data between computers / HMI software, instruments (RTUs) and Modbus-compatible sensors. The CR800 communicates via Modbus over RS-232, RS-485, and TCP. Modbus systems consist of a master (PC), RTU / PLC slaves, field instruments (sensors), and the communications-network hardware. The communications port, baud rate, data bits, stop bits, and parity are set in the Modbus driver of the master and / or the slaves. The Modbus standard has two communications modes, RTU and ASCII. However, CR800s communicate in RTU mode exclusively. Field instruments can be queried by the CR800. Because Modbus has a set command structure, programming the CR800 to get data from field instruments is much simpler than from serial sensors. Because Modbus uses a common bus and addresses each node, field instruments are effectively multiplexed to a CR800 without additional hardware. A CR800 goes into sleep mode after 40 seconds of communications inactivity. Once asleep, two packets are required before the CR800 will respond. The first packet awakens the CR800; the second packet is received as data. CR800s, through DevConfig or the Status table (see the appendix Status Table and Settings (p. 505) ) can be set to keep communications ports open and awake, but at higher power usage. 350