FOUNDATION FIELDBUS H1
The H1 network is most frequently applied to transmitters and valve positioners used for regulatory control. Originally intended to cover the entire fieldbus range from discrete control sensors to PLCs, it has seen less usage in these other areas. For discrete sensors and actuators, it has failed to achieve the low cost point and response speed demanded by those markets. The bus is a powered bus, which runs at 31.25 Kbits/sec. The specification covers power management to allow the bus to be intrinsically safe.
The standard includes a comprehensive User Layer which provides a structure for distributing control function blocks to the field devices, with control of the execution order and data passing between the function blocks is multiple devices.
H1 utilizes a modified token passing protocol to control media access. It has a variety of mechanisms to move data from device to device, as well as between the host and the devices. It categorizes these under Virtual Communication Relationships (VCRs).
The most common of these is called publisher / subscriber, although generically it is more appropriate to consider it to be a producer / consumer multi-cast type. This is used in a cyclic fashion to push data values between function blocks. One of the devices on the bus takes on the role of Link Master which is in charge of the Link Active Scheduler (LAS), which controls the function block execution order and timing . To pass needed parameters from function block to function block, e.g. from an AIN block to a PID block measurement input, it uses the time schedule to cause a Compel Data token to be given to the instrument with the AIN block, which multi-casts the data for use by other function blocks on the H1 bus.
There are also several other communication types (VCRs).
Client / Server – This VCR is used to pass unscheduled data such as tuning parameters, alarm acknowledge, and device upload/download. Example - When the host wishes to send data such as a set point to an instrument, once the host has the Pass Token from the LAS it may act as a client to write a message to the device as a server.
Report Distribution – This VCR is used to pass unscheduled event or trend data to a “group address”. Example – When the field device wishes to send alerts to the host, when it has the token it may initiate a message to the destinations of the group address. Event services of H1 typically use the Report Distribution VCR type. Alert objects allow the reporting of alerts and events from the devices.
View Objects – This access method provides a way to read predefined groupings of parameters from the function blocks. They are typically used to multiple parameters for operator visualization, e.g. to fill in data on a function block detail display. There are 4 defined. View 1 provides the operator dynamic views for set point, process variable, etc.. View 2 provides operator static information such as set point high limit. View 3 contains all dynamic information for a detail display. View 4 provides other static information needed for configuration and maintenance information.
Currently AI, AO, DI, DO, and PID are the only blocks registered by the Foundation, but other block types may be built into devices. FF-891 Part 2 defines a total of 10 function block types. FF-892 Part 3 defines an additional 19 function block types. Parts 4 and 5 define the new MIO (multiple AI, AO, DI, DO) and Flexible Function Blocks. Some vendors of more complex devices, such as PLCs, are expected to utilize the HSE protocol plus the MIO and Flexible Function blocks as a way to bring these types of devices into the Fieldbus Foundation system architecture.
The Fieldbus Foundation sponsored a Fast Ethernet Network Program in late 1997. The resulting HSE specifications define an HSE protocol which can run on commercial Ethernet media, e.g. at 100 Mbps and higher. Redundancy of the HSE network is covered by the specification. The HSE program has served to further thrust the Fieldbus Foundations into the mode of specifying a complete DCS architecture, rather than just a communication bus for field devices.
Core to the specification is a device identified as an HSE Linking Device. These Linking Devices would
- Provide access to the H1 devices
- Bridge between H1 segments on the same Linking Device
- Provide remote bridging to H1 segments on other Linking Devices
- Provide and respond to access initiated by the host / end system.
- In addition to Linking Devices, the specification calls for other types of “nodes” on the HSE network in addition to the host including
- Gateway device – which map I/O from non Foundation networks into Function Blocks
- Ethernet Field Devices – which execute their own function blocks, such as AI AO PID, through the Function Block Application Process (FBAP).
The HSE specification in addition defines additional function block types, such as the Flexible Function Blocks, intended for integration of discrete control or the integration of I/O subsystems.
Field Device Tool (FDT) provides end users with a technology that allows freedom of choice in their selection of field devices and enables standardization of the life cycle management of their plants, while lowering total cost of ownership (TCO). The FDT technology is focused on creating competitive advantage for device manufacturers, while embracing and expanding the utility of their existing device engineering and providing greater opportunity to differentiate device applications.
- FDT is a means for universal engineering configuration and methods access for intelligent devices e.g., Transmitters, Valve positioners, analyzers, MCC, etc.
- FDT provides bus transcendence by allowing the bridging of protocols
- FDT provides higher levels of service functionality by providing a uniform interface independent from the supporting system
- FDT must remain open and a standard implementation
- All company business models must support FDT to lower TCoO
- Value is defined at intelligent device level and/or higher level integrated plant applications
- Utilizing a standard implementation enables vendors to focus on the application, where it belongs.
- Utilizing a standard gives the end user uniform engineering method and security for long-haul
In 1990 the HART foundation was formed to open and promote the HART protocol. It is commonly used as a point to point link between I/O card and instrument at 1200 Kbit/sec. Like 4-20 mA, the instrument power is usually supplied from the I/O card over the instrument wiring. All interface cards must contain both the A/D or D/A converter plus the modem. The analog signal is normally used for the measurement and controller output signals. The Frequency Shift Keyed (FSK) signal is normally used for secondary measurements as well as for passing data to and from the configuration and maintenance software used by the engineers and maintenance personnel. HART is the first digital bus to use the concept of Device Descriptors, DD’s, to achieve interoperability with hosts. These provide information to the configuration, maintenance, and other software tools in the system about the parameters within a transmitter or valve.
PROFIBUS DP is intended for high speed factory automation and building automation. It is most commonly used as a high speed remote I/O bus on controllers designed for factory automation, with direct connectivity to motor drives and other high level discrete control devices. It is not intended to cover the actuators or sensors, but rather bridge to Profibus PA for regulatory control sensors and actuators, and ASI bus for discrete sensors and actuators.
DP is a multi-drop 485 style bus cyclically polled by the master devices on the bus. It supports a variety of data rates up to 12 Mbits/sec. Because length decreases with speed, most applications run at either the 1.5 Mbps or 6 Mbps rates. At the higher speeds runs of up to 500 meters with up to 4 repeaters are allowed.
DP has released V1 and V2 enhancements to the specification. The extensions provide services for multi-mastership and slave configuration functions.
Profibus is also promoting standards around an initiative called ProfiSafe for bringing emergency shut down or other safety related equipment into the network.
The physical network is identical to that of Foundation Fieldbus H1. The PA profile provides AIN and AOUT function blocks which structure the device data into objects for context oriented access. However, unlike H1, Profibus does not provide for control algorithm execution in the field devices. Hence it does not need the function block scheduling mechanisms of H1, nor the sophisticated communication relationships. Instead PA is a cyclically polled network, with the host as master and the instruments as slaves. The messages themselves follow the same protocol as Profibus DP. Profibus PA was first to adopt an enhanced powering model for intrinsic safety called FISCO (fieldbus intrinsically safe concept). This method of protection and calculation provides more power on the bus, allowing more IS devices per segment. This has forced Foundation Fieldbus to follow suit.
