Introduction To Programmable Logic Controlers, Part 2
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An introduction to programmable logic controllers, part II INTERFACE MODULES •Local and remote interface modules (LIM & RIM): For a typical configuration with 2 drops per channel using LIM & RIM, refer to Fig. 1.These modules enable digital and register input/output racks to be remotely installed from the processor. To establish a remote I/O system, a remote interface module is mounted in the remote rack. The local interface should have up to two communication channels. Each channel may support up to eight remote interface modules.
Local and remote interface (LI and RI) modules allow a programmable controller to communicate with remote I/O devices via an interconnecting dual twisted shielded cable connected between the LI and the RI. Another use for LI is when a processor has no onboard registers. In this case, the registers that the processor must address have to come from a local interface module. Also, where a processor can address more registers than are residing on board, the LI gives these registers to the processor.For typical indicators and wiring strips refer to Fig. 2.
These modules can be specified by giving their properties like the power consumed for each local and remote unit, the maximum number of channels per local interface (eg. 2), the maximum number of registers per channel, the maximum number of digitl i/o per channel, the maximum number of drops per channel (eg. 8). The following data may proove to be useful for each drop: the total number of digital i/o and the maximum number of registers. The compatability of these modules with the PLC, i/o
modules and the racks is always required. Command ports are characterized by the transmission rate (eg. 31 Kbaud), the communication mode (eg. full duplex), the physical layer information (like data encoding differential , serial, functional, procedural, electrical, mechanical properties RS 422). For local remote connections refer to Fig. 3.
A mean is provided to set the address of each drop on each channel, like dip switches. The medium of communication between the LIM and the RIM may be dual twisted pair or shielded wire. In general, the LIM receives instructions from the CPU. The LIM then serializes this information and transmits it to the appropriate remote interface module. This is done on one of the independent communication channels. The RIM interprets this, verifies, acknowledges it and then acts upon it. The image table in each of the LIM and RIM will store this information. Now, the exchange of the information between the LIM and RIM is independent of the CPU scan. Transmitted information may have any or the combination of commands to digital i/o, storage register or for housekeeping purposes. Usually, the LIM module would store the control register information, while the RIM would store its own specific drop control information. According to the control information stored in the RIM (per the user program), in an event of a failure in any drop, the running drops may continue their operation (last state condition maintained) or to shut down. •Local and remote transfer interface modules: for critical applications, the transfer interface system will decrease system downtimes by enabling a redundant processor to assume control of the input and output system. This allows a system to operate despite faulty conditions of either the primary or back up processor rack. The remote transfer interface and local transfer interface modules provide the communication between processors and I/O and supervise the actual transfer of control from a faulty primary process to a backup processor and change the designation of the last to primary. Each local transfer interface module has two communiction channels a register transfer channel and an input/output channel. The remote transfer interfaces have two communication channels one between the input/output system and the primary processor the other to the backup processor local transfer interfaces. Refer to Fig. 4 for a transfer interface system.
The specifications for such devices may include the following: the power consumption per interface module (to be included when sizing the power supply of the rack), the number of registers available for external drops and internal data storage, the number of i/o channels per local transfer interface (eg. 1), the maximum number of registers, drops,and digital i/o points, the maximum number of registers and digital i/o per drop, the available i/o points and analog i/o per LTI and the maximum number of LTIs per system, the number of rgister transfer channels per LTI, the communication method, the transmission rate, the transmission distance, the update times under the different possible configurations. Refer to Fig. 5 for typical front indicators of RTIs and LTIs, also, the external connections are shown.
The two possible transfers are the bumpless and the bump. The former is this that occurs when the backup CPU is completely synchronized with the primary processor. I/o states, data values and processor scans are synchronized in both units (no sudden change in the i/o status when the transfer takes place). The latter occurs when sychronization has been lost between the backup and primary processors. Severe noise and broken communication cables can cause loss of synchronization. If a transfer is to happen while the backup and primary processors are out of sync., some i/o values may change when the backup processor takes over and this is a bump in the transfer. The backup processor can be programmed to shut down, if it loses or runs out of sync. with the primary. The register transfer channel is responsible for the transferring of the internal registers contents from the primary LTIs to the backup ones, synchronization of the processors scans and the determination to switch from primary to backup. •Network interface modules. This is a communication interface that allows processors, CRTs, printers, computers and other related devices to communicate with each other, exchanging registers and I/O status. Several networks can be linked together which provides expansion capabilities and allows for distributed networks and network redundancy. These modules can be plugged into any register slot in
any rack. The network interface module will have indicator lights, communication ports, the network port, thumbwheel or rocker or dip switches to indicate NIM address and cosequently communication ports addresses, these switches are also used to indicate the ports baud rate mode of operation of communication ports and the receive of broadcast messages. The major accessories that may be needed to complete a connection to a NIM are: tee connectors (1 per NIM), terminators (2 per network), cable extension (as required). For a front view of a typical NIM refer to Fig. 6.
When power is applied to the NIM ,it will start its self test sequence. In general, this procedure will test the internal PROM memory, clears the RAM memory and performs read/write tests on the RAM. Any message received by the NIM during power up will be ignored and the processor, CRT or any other initiating devices will receive an error. A power down will cause the module to get into a shut down sequence, all communications will come to a halt, the network port will be turned off (disconnected from the network). These modules may handle two types of communications: network (NIM to NIM) and communication ports (PC to PC, CRT to PC,...). The NIM may indicate the priority level of the module, when and for how long it can seize the medium to transmit the message or messages it has. There are a few approaches to access the medium (medium access control protocols). In general, these methods can be classified into distributed and centralized control; another method of classification is whether the protocol is of the contention type (eg. carrier sense medium access control with collision detection for bus topology, slotted ring for ring topology) or the round robin (eg. token bus, token ring) or reservation. MAC protocols may or may not have a provision for setting priorities of te messages to be transmitted from one station to another. For example, the token bus protocol per IEEE 8802.4, the data are prioritized into 4 categories 6,4,2,0. Category 6 being the highest priority, and this priority method works in conjunction with timers to allow a station a maximum time of seizing and transmitting on the medium. The ring topology with the token ring protocol per IEEE 8802.5 has in the access control field of the MAC packet data unit 6 bits to indicate the priority and reservation requirements for each packet (message) to be put on the network. For further details regarding local area network, refer to "An introduction to local area networks" •Network transfer interface modules: Some critical applications demand a control system with redundant capability. A redundant control system has a pair of controllers a primary and a backup. Should the primary controller fail, the backup controller should immediately assume I/P/O/P system control and status. If other devices require information from the redundant controller pair, the devices
must be connected to a communications network. Figure 4 shows a redundant control system configuration.
The TNIM pair will maintain the same communication route of the primary controller before and after the transfer. The benefits of such systems are: fewer communication instructions for any device on the network that communicates to the transfer system controllers. The fiels (connected) devices do not need to determine which is the primary and which is backup controller, the need for different communication rungs are eliminated. Communication instructions for the primary and backup controllers can be identical, thus simplifying the control programs. When the primary controller fails, a flag is set in the backup controller instructing it to assume primary control. The new primary controller informs its associated TNIM to switch to primary status and the primary TNIM to backup status. Another method to initiate a transfer is for the backup system to poll the primary system on a periodic basis for failure. When a failure is detected, the backup controller assumes primary functions and directs the backup TNIM to switch to primary status. In all cases the transfer between the primary and backup TNIM is initiated under the supervision of the controller which assumes primary status. •Multimedia network interface modules: with this module the PLC will be able to communicate via leased and private carrier media. These types of modules could be configrued to interface between a local area network (LAN) and processors, programmers, data log controllers and computers or between two independent LAN environments. For a typical front panel layout refer to Fig. 6. Fig. 9 will show the Network to network mode via cable and via microve tranceiver/antena.
Local and remote interface (LI and RI) modules allow a programmable controller to communicate with remote I/O devices via an interconnecting dual twisted shielded cable connected between the LI and the RI. Another use for LI is when a processor has no onboard registers. In this case, the registers that the processor must address have to come from a local interface module. Also, where a processor can address more registers than are residing on board, the LI gives these registers to the processor.For typical indicators and wiring strips refer to Fig. 2.
These modules can be specified by giving their properties like the power consumed for each local and remote unit, the maximum number of channels per local interface (eg. 2), the maximum number of registers per channel, the maximum number of digitl i/o per channel, the maximum number of drops per channel (eg. 8). The following data may proove to be useful for each drop: the total number of digital i/o and the maximum number of registers. The compatability of these modules with the PLC, i/o
modules and the racks is always required. Command ports are characterized by the transmission rate (eg. 31 Kbaud), the communication mode (eg. full duplex), the physical layer information (like data encoding differential , serial, functional, procedural, electrical, mechanical properties RS 422). For local remote connections refer to Fig. 3.
A mean is provided to set the address of each drop on each channel, like dip switches. The medium of communication between the LIM and the RIM may be dual twisted pair or shielded wire. In general, the LIM receives instructions from the CPU. The LIM then serializes this information and transmits it to the appropriate remote interface module. This is done on one of the independent communication channels. The RIM interprets this, verifies, acknowledges it and then acts upon it. The image table in each of the LIM and RIM will store this information. Now, the exchange of the information between the LIM and RIM is independent of the CPU scan. Transmitted information may have any or the combination of commands to digital i/o, storage register or for housekeeping purposes. Usually, the LIM module would store the control register information, while the RIM would store its own specific drop control information. According to the control information stored in the RIM (per the user program), in an event of a failure in any drop, the running drops may continue their operation (last state condition maintained) or to shut down. •Local and remote transfer interface modules: for critical applications, the transfer interface system will decrease system downtimes by enabling a redundant processor to assume control of the input and output system. This allows a system to operate despite faulty conditions of either the primary or back up processor rack. The remote transfer interface and local transfer interface modules provide the communication between processors and I/O and supervise the actual transfer of control from a faulty primary process to a backup processor and change the designation of the last to primary. Each local transfer interface module has two communiction channels a register transfer channel and an input/output channel. The remote transfer interfaces have two communication channels one between the input/output system and the primary processor the other to the backup processor local transfer interfaces. Refer to Fig. 4 for a transfer interface system.
The specifications for such devices may include the following: the power consumption per interface module (to be included when sizing the power supply of the rack), the number of registers available for external drops and internal data storage, the number of i/o channels per local transfer interface (eg. 1), the maximum number of registers, drops,and digital i/o points, the maximum number of registers and digital i/o per drop, the available i/o points and analog i/o per LTI and the maximum number of LTIs per system, the number of rgister transfer channels per LTI, the communication method, the transmission rate, the transmission distance, the update times under the different possible configurations. Refer to Fig. 5 for typical front indicators of RTIs and LTIs, also, the external connections are shown.
The two possible transfers are the bumpless and the bump. The former is this that occurs when the backup CPU is completely synchronized with the primary processor. I/o states, data values and processor scans are synchronized in both units (no sudden change in the i/o status when the transfer takes place). The latter occurs when sychronization has been lost between the backup and primary processors. Severe noise and broken communication cables can cause loss of synchronization. If a transfer is to happen while the backup and primary processors are out of sync., some i/o values may change when the backup processor takes over and this is a bump in the transfer. The backup processor can be programmed to shut down, if it loses or runs out of sync. with the primary. The register transfer channel is responsible for the transferring of the internal registers contents from the primary LTIs to the backup ones, synchronization of the processors scans and the determination to switch from primary to backup. •Network interface modules. This is a communication interface that allows processors, CRTs, printers, computers and other related devices to communicate with each other, exchanging registers and I/O status. Several networks can be linked together which provides expansion capabilities and allows for distributed networks and network redundancy. These modules can be plugged into any register slot in
any rack. The network interface module will have indicator lights, communication ports, the network port, thumbwheel or rocker or dip switches to indicate NIM address and cosequently communication ports addresses, these switches are also used to indicate the ports baud rate mode of operation of communication ports and the receive of broadcast messages. The major accessories that may be needed to complete a connection to a NIM are: tee connectors (1 per NIM), terminators (2 per network), cable extension (as required). For a front view of a typical NIM refer to Fig. 6.
When power is applied to the NIM ,it will start its self test sequence. In general, this procedure will test the internal PROM memory, clears the RAM memory and performs read/write tests on the RAM. Any message received by the NIM during power up will be ignored and the processor, CRT or any other initiating devices will receive an error. A power down will cause the module to get into a shut down sequence, all communications will come to a halt, the network port will be turned off (disconnected from the network). These modules may handle two types of communications: network (NIM to NIM) and communication ports (PC to PC, CRT to PC,...). The NIM may indicate the priority level of the module, when and for how long it can seize the medium to transmit the message or messages it has. There are a few approaches to access the medium (medium access control protocols). In general, these methods can be classified into distributed and centralized control; another method of classification is whether the protocol is of the contention type (eg. carrier sense medium access control with collision detection for bus topology, slotted ring for ring topology) or the round robin (eg. token bus, token ring) or reservation. MAC protocols may or may not have a provision for setting priorities of te messages to be transmitted from one station to another. For example, the token bus protocol per IEEE 8802.4, the data are prioritized into 4 categories 6,4,2,0. Category 6 being the highest priority, and this priority method works in conjunction with timers to allow a station a maximum time of seizing and transmitting on the medium. The ring topology with the token ring protocol per IEEE 8802.5 has in the access control field of the MAC packet data unit 6 bits to indicate the priority and reservation requirements for each packet (message) to be put on the network. For further details regarding local area network, refer to "An introduction to local area networks" •Network transfer interface modules: Some critical applications demand a control system with redundant capability. A redundant control system has a pair of controllers a primary and a backup. Should the primary controller fail, the backup controller should immediately assume I/P/O/P system control and status. If other devices require information from the redundant controller pair, the devices
must be connected to a communications network. Figure 4 shows a redundant control system configuration.
The TNIM pair will maintain the same communication route of the primary controller before and after the transfer. The benefits of such systems are: fewer communication instructions for any device on the network that communicates to the transfer system controllers. The fiels (connected) devices do not need to determine which is the primary and which is backup controller, the need for different communication rungs are eliminated. Communication instructions for the primary and backup controllers can be identical, thus simplifying the control programs. When the primary controller fails, a flag is set in the backup controller instructing it to assume primary control. The new primary controller informs its associated TNIM to switch to primary status and the primary TNIM to backup status. Another method to initiate a transfer is for the backup system to poll the primary system on a periodic basis for failure. When a failure is detected, the backup controller assumes primary functions and directs the backup TNIM to switch to primary status. In all cases the transfer between the primary and backup TNIM is initiated under the supervision of the controller which assumes primary status. •Multimedia network interface modules: with this module the PLC will be able to communicate via leased and private carrier media. These types of modules could be configrued to interface between a local area network (LAN) and processors, programmers, data log controllers and computers or between two independent LAN environments. For a typical front panel layout refer to Fig. 6. Fig. 9 will show the Network to network mode via cable and via microve tranceiver/antena.
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