Foxboro FBM208/b I/O Module

Foxboro FBM208/b I/O Module

DESCRIPTION

Single input/single output control schemes with high availability requirements can take advantage of the FBM208 arranged as a redundant pair. A readback feature is provided that detects failures of the output channel. In a repair situation, when a single module is replaced, the inputs and outputs are maintained by the redundant module. HART® signals are electrically compatible with the 4-20 mA signals, permitting the plant to upgrade field devices without the need to change the control system. The FBM208b provides the same functionality and is used to upgrade from the older FBM05.

A redundant pair of the modules combine to provide redundancy at the Fieldbus Module (FBM) level, with field I/O wired to one common termination assembly. Each module independently attempts to hold the output(s) at its specified output value(s), and each independently reports its observed value of the inputs.

A redundant analog input and redundant analog output block in the Foxboro™ DCS Control Software validates each input and output in conjunction with information to/ from the module.

Features

• Four 0 to 20 mA dc analog input channels 

• Four 0 to 20 mA dc analog output channels with readback 

• Compares FBM’s output value with the current readback value. If the readback value differs from the desired output by more than ± 2%, the FBM marks its output channel BAD 

• Monitors FBM’s internal channel current loop power and marks the channel BAD if power is less than 16 V dc 

• Marking a channel Bad causes the corresponding channel in the redundant module to continue to drive the outputs 

• Redundant modules increase reliability 

• Termination Assemblies (TAs) for locally or remotely connecting field wiring to the FBM208

Readback

The FBM compares its output value with the current readback value. If the readback value differs from the desired output by more than ±2%, the FBM marks its output channel BAD. In addition, if the output value is greater than the desired output by more than +2%, the power to that channel is shut off to help prevent the bad channel from interfering with the control of that channel by the redundant partner FBM. The power to a failed channel remains off until the FBM is replaced or rebooted by the user.

When the FBM’s output channel is marked BAD, the CP presents that information to the Foxboro DCS for display as a System Management alarm message and as a control block alarm.

Redundant Analog Outputs

Each output channel drives an external load and produces a 0 to 20 mA output.

A redundant analog output block, AOUTR, is used for each redundant pair of outputs. The AOUTR block handles output writes and initialization logic for the redundant channels. On each execution cycle of the AOUTR block, identical output writes are sent to both modules, fully exercising the fieldbus and the logic circuitry of each module. When a failure is detected in one of the modules, its output is driven to 0 mA and the corresponding channel in the good module automatically continues supplying the proper current to the output current loop.

Configurable options in the modules for output failsafe and fallback options are always set to their default values (0 mA). This removes one of the pair of redundant output channels from service for detectable problems such as a module not properly receiving output writes or not passing diagnostic tests on FBM microprocessor writes to output registers. Using the default failsafe/fallback options of 0 mA output also minimizes the possibility of a “fail high” result.

High Accuracy

For high accuracy, the module incorporates sigma-delta converters for each channel. This provides new analog input readings every 25 ms and a configurable integration period to remove any process noise and power line frequencies. Each time period, the FBM converts each analog input to a digital value, averages these values over the time period and provides the averaged value to the controller.

High Reliability

The redundancy of the module pair, coupled with the high coverage of detected faults, provides a very high subsystem availability time. The microprocessor of each module executes the analog I/O application program, plus diagnostic routines that validate the health of the FBM. Either module may be replaced without upsetting field input or output signals to the good module. The module can be removed/replaced without removing field device termination cabling, power, or communications cabling. Light-emitting diodes (LEDs) incorporated into the front of the module provide visual indications of the module’s operational status.

Functional Specifications

Input Channels (Four)

• Analog Accuracy (Includes Linearity): ◦ ±0.3% of span which Includes the tolerance of the sense resistor in the redundant adapter. 

◦ Accuracy temperature coefficient: ±50 ppm/ºC. 

• Input Channel Impedance: 60 Ω nominal (resistor in redundant adapter). 

• Input Signal A/D Conversion: Each channel performs its own A/D signal conversion, using the sigma-delta conversion technique. 

• Integration Period: Software configurable 

• Common Mode Rejection: >100 db at 50 or 60 Hz 

• Normal Mode Rejection: >95 db at 50 or 60 Hz 

• Loop Power Supply Protection: Each channel is channel-to-channel galvanically isolated, current limited, and voltage regulated. All analog inputs are limited by their design to less than 30 mA. If the current limit circuit shorts out, the current is limited to about 100 mA. 

• Field Device Cabling Distance: Maximum distance of the field device from the FBM is a function of compliance voltage (22.8 V dc), wire resistance, and voltage drop at the field device.