MODBUS / SCADA

A RTU, or Remote Terminal Unit is a device which interfaces objects in the physical world to a DCS or SCADA system by transmitting telemetry data to the system and/or altering the state of connected objects based on control messages received from the system.

A typical RTU has a communications interface (usually serial, Ethernet, proprietary, or any combination), a simple processor, some environmental sensors, some override switches, and a bus which it uses to communicate with devices and/or interface boards. This bus is sometimes called a device bus or a field bus. Standards include the ISO Controller Area Network (ISO 11898), MODBUS, and others. Many vendors provide proprietary buses for their equipment; examples include Allen-Bradley's Data Highway and HSQ Technology's MISERnet. Sometimes a device or field bus standard can be used to interconnect RTUs and host systems as well as field devices and RTUs.

Interface boards come in analog and digital flavors, and are typically designed for input only, output only, or both. These main types of interface boards are often abbreviated as "DI" (digital input), "AO" (analog output), and so forth. These categories are further subdivided based on the range of inputs, the amount of protection against voltage surges, and the amount of intelligence on the interface board. Sometimes an RTU or PLC has integral interfaces used to allow the deployment of only a RTU (without a bus and interface modules) to monitor or control just a few devices.

The interface boards are in turn connected to physical objects using wires. A typical application found in most SCADA implementations is to connect high current capacity relays to a digital output (or "DO") board to switch power on and off to devices in the field. The DO board switches voltage to the coil in the relay, which closes the high current contacts, which completes the power circuit to the device. Analog inputs are usually 24 V with a current range between 4 and 20 milliamperes (4-20mA); the RTU or host system then translates this into the appropriate units such as gallons of water left or temperature before presenting the data to the user via the HMI or MMI.

Modern RTUs are usually capable of executing simple programs autonomously without involving the host computers of the DCS or SCADA system to simplify deployment, and to provide redundancy for safety reasons. A RTU in a modern water management system will typically have code to modify its behavior when physical override switches on the RTU are toggled during maintenance by maintenance personnel. This is done for safety reasons; a miscommunication between the system operators and the maintenance personnel could cause system operators to mistakenly enable power to a water pump when it is being replaced, for example.

RTUs and PLCs are increasingly beginning to overlap in responsibilities, and many vendors sell RTUs with PLC-like features and vice versa. The industry has standardized on the IEC 61131-3 functional block language for creating programs to run on RTUs and PLCs, although nearly all vendors also offer proprietary alternatives and associated development environments.

All M-series Super PLCs features two independent serial ports (1 RS232 and 1 RS485) that speak both MODBUS ASCII and RTU protocols fluently! These super PLCs can work happily with many other third party MODBUS masters or slave devices or with other super PLCs, thereby providing an open system solution. It is ready to interface to vast majority of other industrial software and hardware as described below.

1. SCADA Software Interface The SCADA is the MODBUS master and the PLC works as the slave.	Most of the well known SCADA (Supervisory Control and Data Acquisition) software do come with MODBUS drivers which can immediately and painlessly interface with the super PLCs. The following brands are some of those that are either tested or known to work with the super PLCs: 1. National Instrument's Lookout 2. Elipse Software 3. DAQFactory (from $199) 4. Wonderware 5. Intellusion 6. GE Fanuc Cimplicity
2. Human Machine Interface (HMI) The HMI is the MODBUS master and the PLC works as the slave. The PLC's serial port is fully configurable from 300 to 500kbps, 7/8 data bit, 1/2 stop bits. odd/even/no parity and should work with most HMI on the market.	HMIs are mostly standalone graphical LCDs with touch-screen display. The HMI are mostly designed to work with PLCs using RS232 or RS485 and most of them do support the MODBUS protocols. The follow HMI are either tested or known to work with the super PLCs 1. Maple Systems 2. Taian HMI 3. Optimation HMI 4. GE-Fanuc Quickpanel 5. Pro-face 6. Hakko 7. EXOR HMI (Italy)
3. Controlling External RTU Devices	Any M-series Super PLC can also act as the MODBUS master controlling other MODBUS ASCII or RTU slave devices. This is accomplished incredibly easily using the built-in READMODBUS and WRITEMODBUS commands supported by TBASIC. The ability of the Super PLC to work as MODBUS host eliminates the need to use a host computer just to collect data from a few MODBUS or RTU devices. The T100MD1616+ can take the place of the host computer for a much lower cost of deployment, more compact and much more reliable in the hostile industrial environment.

The Super PLC's Dual Independent MODBUS Ports

Amazingly, even the lowest cost T100MD888+ super PLC comes with two independent UARTs for its two serial ports: COMM1 is an RS232 port and COMM3 is an RS485 port. Both comm ports support MODBUS ASCII and RTU and can run simultaneously! The built-in RS485 port means networking a number of super PLCs will cost you extra $0.00 - (except maybe only for the price of the twisted pair cable). You can therefore connect as many as 32 super PLCs to a SCADA host very cost effectively. By replacing the standard RS485 driver with 1/4 or 1/8 power type (e.g. Linear Technology LTC1487) as many as 256 PLCs can be networked together and pay next to nothing!

Another possible application is to use the RS485 for inter-networking between the PLCs so that data can be shared between the PLCs. You can use the MODBUS protocol or the native protocol to accomplish this networking function. Hence you can have a network of super PLCs in a big system (such as building automation control) networked together by the RS485 , at the same time each PLC node can talk to other MODBUS host or slave devices using its RS232 port. No other PLCs in the market today can rival the vast communication flexibility of the M-series super PLCs regardless of their price! This makes the T100MD1616+ an ideal choice for all applications from standalone machine controller to large systems employing hundreds of PLCs