1. Introduction
The TRICON is a programmable logic controller that provides a high level of fault tolerance
Fault tolerance is the ability to detect errors and to take appropriate corrective action on-line.
With fault tolerance, there is an increase in safety and in availability of the controller and the process.
The TRICON is used in critical process applications such as ESD and F&G systems
It provides fault tolerance through Triple Modular Redundant (TMR) architecture, which consists of three identical legs; each leg independently executes an application in parallel with the other two legs.
A 2oo3 hardware/software voting mechanisms qualify and verify all digital inputs and outputs; analog inputs are subject to a mid-value selection process
If a hardware failure occurs in one leg, the faulty leg is overridden by the other legs.
A faulty module can easily be replaced while the TRICON is on-line and without process interruption; the system then reconfigures itself to full TMR operation
Setting up applications is simple because the triplicated system operates as one control system
2. Theory of operation
The TRICON is designed with a fully triplicated architecture throughout, from the input modules through the Main Processors to the output modules.
Every I/O module houses the circuitry for three independent legs.
Each leg on the input modules reads the process data and passes that information to its respective Main Processor.
Then input data is voted at the Main Processors before processing
Once per scan, the three Main Processors communicate with each other using a proprietary high-speed bus system called the TRIBUS.
The Main Processors execute the application and send outputs generated by the application to the output modules.
The TRICON votes the output data on the output modules before acting on final control element in the field.
For each I/O module, the system can support an optional hot-spare module which takes control if a fault is detected on the primary module during operation.
Triplicated architecture of TRICON controller
3. Main Processor Modules
A TRICON contains three Main Processor (MP) modules to control three separate legs of the system; each Main Processor operates in parallel with the other two Main Processors
A dedicated I/O communication processor (IOPCOMM) on each Main Processor manages the data exchanged between the Main Processor and the I/O modules.
A triplicated I/O bus is extended from chassis to chassis by means of I/O Bus cables.
As each input module is polled, the new input data is transmitted to the Main Processor over the appropriate leg of the I/O Bus.
The input data from each Input Module is assembled into a table in the Main Processor and stored in memory for use in the hardware voting process
The individual input table in each Main Processor is transferred to its neighboring Main Processors over the TRIBUS.
During this transfer, hardware voting takes place.
If a disagreement occurs, the signal value found in two out of three tables prevails, and the third table is corrected accordingly.
After the TRIBUS transfer and input data voting have corrected the input values, these corrected values are used by the Main Processors as input to the user-written application.
The application is developed in the TRISTATION and downloaded to the Main Processors.
The 32-bit main microprocessor executes the user-written application in parallel with the neighboring Main Processor modules.
The user-written application generates a table of output values based on the table of input values, according to the rules built into the application by the customer.
The I/O processor on each Main Processor manages the transmission of output data to the output modules by means of the I/O bus.
4. Bus Systems, Power Distribution & field signals
Three triplicated bus systems are etched on the chassis backplane:
- The TRIBUS
- The I/O bus
- The communication bus.
The TRIBUS consists of three independent serial links operating at 25 Mbaud.
It synchronizes the Main Processors at the beginning of a scan.
Then each Main Processor sends its data to its upstream and downstream neighbors.
The 375 Kbaud triplicated I/O bus transfers data between the I/O modules and the Main Processors.
The 2 Mbaud communication bus (COMM) runs between the Main Processors and the communication modules.
Each module in the chassis draws power from both power rails through dual power regulators.
Main chassis backplane
Field Signals
Each I/O module transfers signals to or from the field through its associated field termination assembly.
Two positions in the chassis tie together as one logical slot.
The first position holds the active I/O module and the second position holds the hot-spare I/O module.
Each connection extends from the termination module to both primary and hot-spare I/O modules.
Therefore, both the active module and the hot-spare module receive the same information from the field termination wiring.
5. Main I/O modules
· Digital Input
Each of the three legs measures the input signal and passes the result, using an I/O communication microprocessor, to its associated Main Processor.
Then input data is voted at the Main Processors before processing.
· Digital Output
Each of the three legs includes an I/O microprocessor which receives its output value from the I/O communication processor on its corresponding Main Processor.
The modules use a circuitry which votes on the individual output signals just before they are applied to the load.
· Analog Input
Each of the three legs measures the input signal and passes the result to its associated Main Processor.
The middle value or the average value is selected by each Main Processor and then used by the application.
· Analog Output
This module receives three output values, one for each leg from the corresponding Main Processors; each leg has its own DAC
One of the three legs is selected to drive the analog output
The “driving leg” is rotated among the legs to use all three legs.
If a fault occurs in the driving leg, a new leg is selected to drive the field device
6. Communication Modules
· Enhanced Intelligent Communication Module (EICM):
Supports the industry-standard Modbus protocol
User can select; RS-232 point to point interface (one master / one slave) or RS 485 multipoint interface (one master / up to 32 slaves) serial communication with external devices at a speed up to 19.2 kbaud
A TRICON supports up to two EICMs in one logical slot, each EICM has four serial ports and one parallel port which can operate concurrently.
The four serial ports are uniquely addressed and support either the Modbus or TRISTATION interface,the parallel port provides a Centronics interface to a printer.
· Network Communication Module (NCM)
Supports IEEE 802.3 networking over a 10 Mbit/s data link for the use of:
- TRICONEX applications
- User-written applications using the TRICON System Access Application (TSAA) protocol
- “Open” networking with external systems by means of TCP/IP – UDP/IP protocol
- TRICONEX peer to peer and time synchronization protocol
· Safety Manager Module (SMM)
The SMM acts as an interface between a TRICON controller and Honeywell's TDC 3000 Universal Control Network (UCN)
The SMM communicates process information at full network data rates
· Advanced Communication Module (ACM)
The ACM acts as an interface between a TRICON controller and Foxboro's Intelligent Automation I/A Series DCS Network (NODEBUS)
The SMM communicates process information at full network data rates
7. TRICONEX Protocols and Applications
A protocol is a set of rules for exchanging data between two or more devices.
TRICONEX has developed one peer-to-peer protocol and three master/slave protocols (time synchronization, TRISTATION, and TSAA) to support different types of applications.
· Peer-to-Peer protocol
Any device on the network can initiate a data transfer operation.
It allows TRICONS to exchange small amounts of safety and process information on the proprietary network.
· Master/slave protocol
Only the master device can initiate a data transfer operation.
- Time Synchronization
It is used to maintain a consistent time base for all TRICONS connected to each other by means of NCMs.
- TRISTATION
The master (the TRISTATION PC) communicates with the slave (TRICON) over an 802.3 network.
The master can communicate with only one slave at a time.
- TRICON System Access Application (TSAA)
The master (external host) communicates with one or more slaves (TRICONS) over an open network.
8. System configuration
A TRICON is composed of a Main Chassis and up to 14 Expansion or Remote Expansion (RXM) Chassis.
The maximum system size is 15 chassis supporting a total of 118 I/O modules and communication modules
· Chassis Layout
The Main chassis consists of:
Two Power Supplies
Three Main Processors
Six logical slots for I/O and communication modules
One COM slot with no hot-spare position.
Each logical slot provides two physical spaces for modules, one for the active module and the other for its optional hot-spare module.
The layout of an Expansion Chassis is similar to that of the Main Chassis, except that Expansion Chassis provide eight logical slots for I/O modules.
· I/O Bus
The Main and Expansion Chassis are interconnected by a triplicated RS-485 I/O Bus cables.
Expansion chassis are used when I/O Bus cables length is up to 30 m
Remote Expansion (RXM) Chassis enable a system to extend to remote locations up to 12 km.
· Online Module Repair
The TRICON chassis provides two approaches, the hot- spare method and the online module replacement.
With the hot-spare method, a logical slot contains two identical I/O modules.
The primary is active, and the other the hot-spare is powered but inactive.
If a fault is detected on the primary module, the TRICON automatically switches control to the hot-spare module, allowing the system to maintain three healthy legs continuously.
The faulty module can then be removed and replaced.
A module can be replaced online even when only one I/O module is normally installed in a logical slot.
If a fault occurs, the FAULT indicator turns on, but the module remains active on two legs.
A replacement module is then inserted into the unused space in the slot.
The TRICON will grant control to this second I/O module after it passes a diagnostic test. Once the replacement I/O module becomes active, the faulty I/O module can be removed.
1) COM slot can be configured for EICM or NMC only
Layout of a TRICON chassis
· I/O Bus Connections
Each chassis has three sets of RS-485 I/O Bus Ports (IN & OUT).
Additional chassis may branch out from the Main Chassis by means of the I/O Bus Ports, up to a maximum of 14.
There are six ports, two sets of three for triplicated serial communication.
One set of three I/O Bus cables is required for each Expansion Chassis, and for each RXM Chassis that houses a Primary RXM module set.
Remote RXM Chassis are connected to the Primary RXM Chassis with fiber-optic cables.
Communication across the I/O Bus cables (and the RXM fiber-optic cables) proceeds at 375 kBaud, the same rate as the internal I/O bus on each chassis's backplane.
· Key switch
The Main Chassis has a four-position key:
RUN
Normal operation with read-only capability.
The Main Processors execute the previously-loaded application
PROGRAM
For program loading and checkout.
Allows control of the TRICON from the TRISTATION
STOP
Stops reading inputs, forces non-retentive digital and analog outputs to 0, and halts the application.
Retentive outputs retain the value they had before the key switch was turned to STOP.
REMOTE
Allows writes to program variables by TRISTATION and external hosts
9. TRISTATION 1131 Developer's Workbench
The TRISTATION is an integrated tool for developing, testing and documenting process applications for the TRICON.
The software runs under Microsoft's Windows NT operating system.
Any PC that is compatible with the NT operating system can be used for TRISTATION.
TRISTATION provides three editors to support the IEC 61131-3 languages; Function Block Diagram, Ladder Diagram and Structured Text.
The Workbench also provides the Programming Language Editor (CEMPLE) that TRICONEX developed to support the widely used Cause and Effect Matrix
Programming Languages
Function Block Diagram (FBD)
A graphical language that corresponds to circuit diagrams.
FBD elements appear as blocks that are wired together, to form circuits; the wires transfer data between elements.
Structured text (ST)
A high-level, textual language, that is similar to PASCAL.
ST allows users to create Boolean and arithmetic expressions, and programming structures such as conditional (IF, THEN, ELSE) statements.
Ladder diagram (LD)
A graphical language that uses a standard set of symbols for representing relay logic.
The basic elements are coils and contacts, which are connected by links
Cause and Effect Matrix Programming Language Editor (CEMPlE)
A high-level graphical language that provides a two-dimensional matrix in which the user can easily associate a problem in a process with one or more corrective actions.
The problem is known as the cause and the action as the effect.
The matrix associates a cause with an effect in the intersection of the cause row and the effect column.
Glossary
Availability
The probability that the control system is operational at some instant of time
CEM
Stands for Cause and Effect Matrix which is a two-dimensional matrix for the development of safety applications.
In this type of matrix, causes are represented by rows and effects are represented by columns.
CE Mark
A type of certification by the European Union which ensures the electro-magnetic compatibility of TRICONEX controllers with other pieces of electrical and electronic equipment.
CEMPLE
A language editor in the TRISTATION 1131 Developer's Workbench that allows you to develop CEMs for safety shutdown applications.
Communication modules
Modules that enable the TRICONEX to communicate with host computers.
TRICONEX offers communication modules which use IEEE 802.3, TCP/IP or Modbus protocol.
Fault tolerance
The ability to identify and compensate for failed control system elements and allow repair while continuing an assigned task without process interruption.
Fault tolerance is achieved by incorporating redundancy and fault masking.
Hot spare
A unique feature of TRICONEX controllers which allows spare I/O modules to be installed with automatic switch to the spare in case the primary module fails.
IEEE
Stands for the Institute of Electrical
Intermittent fault
A fault or error that is only occasionally present due to unstable hardware or varying software states.
Logical slot
In a TRICONEX chassis, a logical slot is a repository for a primary module, a hot spare module, and their associated field termination component
Matrix
1. A CEM program
2. A traditional methodology for ESD applications which associates a problem (cause) in a process with one or more actions (effects) that must be taken to correct the problem.
Peer-to-peer
A protocol that allow multiple TRICONEX controllers on a proprietary network to exchange limited amounts of process and safety information.
Protocol
A set of rules describing the format used for data exchange between two entities.
Reliability
The probability that no failure of the system will have occurred in a given period of time
TCP/IP
Stands for Transmission Control Protocol/Internet Protocol which are protocols for the Transport and Network layers of the OSI network model.
Transient fault
A fault or error resulting from a temporary environmental condition
TSAA
Stands for Tricon System Access Application which is a master-slave protocol in which the master (an external host) communicates with one or more slaves (TRICONEX controllers) over an open network.
UDP/IP
Stands for User Datagram Protocol/Internet Protocol (TCP/IP) which are protocols for the Transport and Network layers of the OSI network model.
Voting
A mechanism whereby each leg of a TMR compares and corrects the data in each leg using 2oo3 majority voting scheme
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