3.1        Communications Overview

There are two aspects to Communications to field devices and controllers that you must consider:

1.      The Physical Interface (e.g. RS-232C, RS422, RS-485, Ethernet, an proprietary networks.

2.      The Protocol (e.g. Modbus RTU, Modbus ASCII, DataHighway Plus).

The Physical Interface (COMPORT). This is the communications hardware (COMPORT) used to connect to the device.  The COMPORT type is described by the  Interface Name used when configuring the WebAccess COMPORT: Serial, TCP/IP, OPC and API.

Most proprietary network cards require an API interface, which is 3rd Party Software Driver for the network card (for example, RSLINX is required to communicate via Allen-Bradley's Control Net). Most Serial and TCP/IP interfaces uses a "native" driver built into WebAccess.

The Serial Physical Interface determines voltage levels and current levels of the electrical signal. RS232-C and RS485 are examples of physical serial interfaces.  COM2, COM3, COM4 are common serial ports on your PC that are usually RS232-C.  These can be relatively easily converted to RS-485 or other physical interfaces.

Physical converters are commonly used to convert long distance serial protocols (like RS-422) to the short haul physical protocol used by most Personal Computers (RS-232 -C).  Companies, like B&B Electronics Mfg Co at http://www.bb-elec.com/ make serial converters. These are Physical converters only. 

TCP/IP uses the network card in your SCADA node (Ethernet is the most common TCP/IP network).

OPC is an industry standard software interface (OLE for Process Control).

Protocol - The DEVICE TYPE determines the Protocol used by WebAccess to communicate with the automation device. The Protocol is software and hardware dependent.  The Protocol determines how the 1 and 0 (the bits) are converted into meaningful numbers and characters.  Modbus RTU is an example of a Serial Communications protocol. 

The protocol of the device and the WebAccess Device Driver must be the same. The protocol is tremendously variable among manufactures. It is almost impossible to convert an Echelon LON protocol into a Siemens S7, for example.  There are exceptions; for example, there are Modbus RTU to Allen-Bradley Data Highway converters.  However, these exceptions are rare. The Protocol is specified when you ADD DEVICES to the comport (see Device Properties).

An outline of the Steps or procedures recommended to configure a SCADA node with communications to PLCs, Controllers, IO and other field deices should include:

1)     Determine the communications Protocol and Device Driver required by field device. For example, Modbus RTU, Modbus Ethernet, Allen Bradley Data Highway Plus, etc.    You cannot easily change the device type later: you would have to create a new device and recreate all the tags in that device.

2)     Determine the physical communications port to be used to connect to the field devices. For example COM1, COM2, Network card using TCP/IP, an OPC Server or a proprietary network card requiring special software (API). You can change the Port type and Port Number later.

If using multiple communications ports, determine which tags are on which comport. It's easy to change the comport number for all the tags on a comport.  It is harder to move only some tags from one comport to another.  However, it is possible to move tags between comports.

3)     Determine the addresses of the device and data in the device (Tag Addresses). For example, Modbus RTU PLC has an Address between 1 and 255.  Analog Inputs have addresses between 30001 and 39999.  You can change addresses of tags later for most device drivers.