Serial Interfaces

Serial is a Serial Communications Port on the PC. Selecting Serial means to access the Comport with the same number.  For example, to use COM2 on the SCADA node, Port2 in the WebAccess Project must be configured as Serial.

This is an RS232-C, RS485 or RS-422 port on the SCADA node: COM1, COM2, COM3, COM4 etc.  For Serial devices, the port number must be the same as the number of the comport (Port 1 will be COM1, port 2 will be COM2).  For most drivers, this must be an IBM standard Serial port. For example, RS-232 or RS-422 UART (Universal Asynchronous Receiver Transmitter) chip with a FIFO (First-In, First-Out) buffer.

Serial type Devices usually have a "native" driver written by WebAccess.  It is possible to have a Serial connection to the device, but use the API or OPC Port Types in WebAccess.

Often this standard Serial link is to a converter, to convert RS-485 to RS232-C. Another use is to a gateway that converts a proprietary physical interface to RS-232C (for example Moore LIL to RS232) 

The protocol, baud rate, data bits, stop bits, and parity settings must match those configured in each field automation device on this comport. For example, all PLCs on the serial cable connected to this port, must communicate using the same protocol, baud rate, data bits, stop bits, and parity settings.  If this is to a gateway, then the protocol, baud rate, data bits, etc must match that of the gateway device.

There are two aspects to Serial Communications that you must consider:

3.      The Physical Interface (e.g. RS-232C, RS422, RS-485, and proprietary)

4.      The Protocol (e.g. Modbus RTU, Modbus ASCII, Moore LIL)

The Physical Interface. 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. 

Protocol - The Protocol is software and hardware dependent.  It 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.  But these exceptions are rare. The Protocol is specified when you ADD DEVICES to the comport (see Device Properties).

See your communications Driver Guide for details about your device.  There is also a reference section in the Appendix on Serial Communications.