FoxBoro IA Overview

The Foxboro I/A DCS is hosted on a proprietary dual redundant control bus called Nodebus. The two buses are designated 'Cable A' and 'Cable B'. It is possible to connect external applications via standard Ethernet to this bus via either of two interface modules:

·      Single Nodebus Interface Unit - often called a MAU (Media Access Unit)

In this case the interface supports reception on both 'Cable A' and 'Cable B' but is setup to transmit on only one of these cables. This setup is achieved via a system of "letterbugs". A letterbug is a small passive device that represents a single alphanumeric value. Every station connected to a Foxboro I/A nodebus is uniquely identified by a set of up to six letterbug devices that are typically plugged into the front panel of a particular station module. The use of letterbugs for a single nodebus interface unit is slightly different. In this case, the interface is configured as either "CA" or "CB". The "C" indicates that it can listen on either cable while the 'A' or 'B' indicates which cable will accept transmissions from an external system. This external system will itself be identified by a six letterbug ID.

·      Dual Nodebus Interface Unit

In this case the interface supports both reception and transmission on both 'Cable A' and 'Cable B'. An RS232C serial link enables transmission to be switched via either 'A' or 'B'. For backward compatibility it is possible to enable this interface to perform like a single nodebus interface by using either "CA" or "CB". The letterbug designation "CC" will enable full featured dual nodebus interfacing - which is the default behaviour.

The DM_FOXIA module may use either interface unit.

Dual nodebus operation of the Single Nodebus Interface unit may be achieved by serial port switching of a relay module that provides a "switchable" letterbug ID whose value may be set to either 'A' or 'B' under software control.