Fieldbus Modules provide connection of digital I/O, analog I/O, and Intelligent Transmitters to control processors. There are two types of Fieldbus Modules: Main and Expansion. Some main modules can be expanded using an expansion module.
A wide range of Fieldbus Modules is available to perform the signal conversion necessary to interface the control processor with field sensors and actuators.
The Control Processor is used in three different configurations, which provide broad flexibility in Fieldbus implementation:
- Local Fieldbus – Used only within the enclosure. Fieldbus Modules attach directly to the redundant local bus.
- Twinaxial (Dual-Conductor Coaxial) Fieldbus Extension (Figure 4.2) – Using twinaxial cable, the Fieldbus can optionally extend outside of the enclosure. Fieldbus Modules attach to the extended bus through Fieldbus isolators. The twinaxial Fieldbus extension may be redundant.
- Fiber Optic Fieldbus Extension – The fiber optic Fieldbus can optionally extend the distance as well as add application versatility and security.
All three Fieldbus configurations use serial data communication complying with Electronic Industrial Association (EIA) Standard RS-485.
Cluster I/O Subsystem Interfacing
The Control Processor interfaces with the Fieldbus Cluster Input/Output Subsystem that consists of the Fieldbus, a multi-slot chassis configuration of a Fieldbus Processor, analog/digital Fieldbus Cards (FBCs), and power supply and power monitor card. These Cluster I/O subsystems meet the needs of applications where a high number of channels per card are required. Figure below shows a typical twinaxial Fieldbus configuration.
Fieldbus Cluster I/O Subsystem
The Fieldbus Cluster Input/Output Subsystem provides full support for analog measurement, digital sensing, and analog or discrete control capabilities. The Subsystem integrates with the Control Processor or Personal Workstation via the Fieldbus, and includes a multi-slot chassis configuration made up of a Fieldbus Processor, Analog/Digital Fieldbus Cards (FBC), subsystem main power supply, and power monitor card.
The Fieldbus Cluster I/O Subsystem is configurable, gathering analog measurements, while simultaneously handling analog and digital input and output channels. The Fieldbus Cluster I/O Subsystem is offered in both non-redundant and redundant configurations. Each in a redundant pair is individually addressable on the Fieldbus with a unique logical address. In a redundant configuration, the FBPs provide switchover from the primary FBP to the redundant FBP and back again automatically. The FBCs are suitable in applications where a high number of channels per card are required. They are ideal for non-isolated and isolated input signal gathering and data acquisition systems where high quantities of “points per cluster” areas are desired. The FBCs may be optionally connected as redundant pairs. Various input cards are available with one of the following three levels of isolation:
- Non-isolated – Each channel is referenced to ground and the card itself is referenced to ground.
- Group-isolated – Electrically separate card-to-card but not channel-to-channel on the same card.
- Isolated – Each channel is electrically separated from any other channel, card, group, building, site, etc.
The Fieldbus Processor (FBP) module provides communication between the Fieldbus Cards (FBCs) and the Control Processor. Optionally available is redundancy for the FBP module. Each FBP module is individually addressable via the Fieldbus. If the primary FBP fails or is taken off-line, the secondary FBP automatically assumes control. It remains in control until the primary FBP returns on-line .
The Fieldbus Cards support a variety of analog and digital I/O signals. The FBCs convert electrical I/O signals used by field devices to permit communication with these devices via the Fieldbus.
The FBCs can be connected in a redundant configuration via the hardware. The redundant FBCs must be in adjacent slots and they are connected via a hardware adapter at the interface to the field devices. In an FBC redundant configuration, the FBP determines which FBC of the redundant pair is to supply the data to the Control Processor. This is done in the software by a predetermined set of conditions.
The analog FBCs support analog signal types and control functions equipped with accurate signal conditioning circuitry, the analog cards interface between process sensors and actuators.
To input an analog voltage (into DCS) the continuous voltage value must be sampled and then converted to a numerical value by an A/D converter. Figure below shows a continuous voltage changing over time. There are three samples shown on the figure. The process of sampling the data is not instantaneous, so each sample has a start and stop time. The time required to acquire the sample is called the sampling time. A/D converters can only acquire a limited number of samples per second. The time between samples is called the sampling period T, and the inverse of the sampling period is the sampling frequency (also called sampling rate). The sampling time is often much smaller than the sampling period.
Analog outputs are much simpler than analog inputs. To set an analog output an integer is converted to a voltage. This process is very fast, and does not experience the timing problems with analog inputs. But, analog outputs are subject to quantization errors. Figure 4.7 gives a summary of the important relationships. These relationships are almost identical to those of the A/D converter. Assume we are using an 8 bit D/A converter that outputs values between 0V and 10V. We have a resolution of 256, where 0 results in an output of 0V and 255 results in 10V. The quantization error will be 20mV. If we want to output a voltage of 6.234V, we would specify an output integer of 159, this would result in an output voltage of 6.235V. The quantization error would be 6.235V-6.234V=0.001V. The current output from a D/A converter is normally limited to a small value, typically less than 20mA.
The digital FBCs consist of 32- and 64-channel types. Inputs can be either voltage monitoring or contact sensing.
Contact inputs must convert a variety of logic levels to the 5Vdc logic levels used on the data bus. This can be done with circuits similar to figure below. Basically the circuits condition the input to drive an optocoupler. This electrically isolates the external electrical circuitry from the internal circuitry. Other circuit components are used to guard against excess or reversed voltage polarity.
Contact outputs must convert the 5Vdc logic levels on the DCS data bus to external voltage levels. This can be done with circuits similar to figure below. Basically the circuits use an optocoupler to switch external circuitry. This electrically isolates the external electrical circuitry from the internal circuitry. Other circuit components are used to guard against excess or reversed voltage polarity.
- 0 to 20 mA Input/Output Interface
- Pulse Input, 0 to 20 mA Output Interface
- Thermocouple/ Millivolt Input Interface
- RTD Input Interface
- High Power Contact/dc Input/Output Interface
Foundation Fieldbus Technology
FOUNDATION fieldbus is an all-digital, serial, two-way communications system that serves as the base-level network in a plant or factory automation environment.
It’s ideal for applications using basic and advanced regulatory control, and for much of the discrete control associated with those functions. Two related implementations of FOUNDATION fieldbus have been introduced to meet different needs within the process automation environment. These two implementations use different physical media and communication speeds.
- H1 works at 31.25 Kbit/sec and generally connects to field devices. It provides communication and power over standard twisted-pair wiring. H1 is currently the most common implementation and is therefore the focus of these courses.
- HSE (High-speed Ethernet) works at 100 Mbit/sec and generally connects input/output subsystems, host systems, linking devices, gateways, and field devices using standard Ethernet cabling. It doesn’t currently provide power over the cable, although work is under way to address this.
Conventional analog and discrete field instruments use point-to-point wiring: one wire pair per device. They’re also limited to carrying only one piece of information — usually a process variable or control output — over those wires. As a digital bus, FOUNDATION fieldbus doesn’t have those limitations.
- Multidrop wiring. FOUNDATION fieldbus will support up to 32 devices on a single pair of wires (called a segment) — more if repeaters are used. In actual practice, considerations such as power, process modularity, and loop execution speed make 4 to 16 devices per H1 segment more typical.
That means if you have 1000 devices — which would require 1000 wire pairs with traditional technology — you only need 60 to 250 wire pairs with FOUNDATION fieldbus. That’s a lot of savings in wiring (and wiring installation).
- Multivariable instruments. That same wire pair can handle multiple variables from one field device. For example, one temperature transmitter might communicate inputs from as many as eight sensors — reducing both wiring and instrument costs.
Other benefits of reducing several devices to one can include fewer pipe penetrations and lower engineering costs.
- Two-way communication. In addition, the information flow can now be two-way. A valve controller can accept a control output from a host system or other source and send back the actual valve position for more precise control. In an analog world, that would take another pair of wires.
- New types of information. Traditional analog and discrete devices have no way to tell you if they’re operating correctly, or if the process information they’re sending is valid.
But FOUNDATION fieldbus devices can tell you if they’re operating correctly, and if the information they’re sending is good, bad, or uncertain. This eliminates the need for most routine checks — and helps you detect failure conditions before they cause a major process problem.
- Control in the field. FOUNDATION fieldbus also offers the option of executing some or all control algorithms in field devices rather than a central host system. Depending on the application, control in the field may provide lower costs and better performance — while enabling automatic control to continue even if there’s a host-related failure.
FOUNDATION fieldbus is covered by standards from three major organizations:
- ANSI/ISA 50.02
- IEC 61158
- CENELEC EN50170:1996/A1
The technology is managed by the independent, not-for-profit Fieldbus Foundation, whose 150+ member companies include users as well as all major process automation suppliers around the globe.
Some suppliers have even donated fieldbus-related patents to the Fieldbus Foundation to encourage wider use of the technology by all Foundation members.
Interoperability simply means that FOUNDATION fieldbus devices and host systems can work together while giving you the full functionality of each component.