Foxboro 0 to 20 mA Output Interface Module FBM44
I/A Series® Hardware
Dual Baud Rate Intelligent Transmitter/
0 to 20 mA Output Interface Module (FBM44)
The Dual Baud Rate Intelligent Transmitter (IT)/0 to
20 mA Output Interface Fieldbus Module contains
four individual Intelligent Transmitter interface
channels and four 0 to 20 mA dc analog output
channels. The module is a main type and
independently connects to the redundant Fieldbus.
Intelligent Transmitter (IT) Interface Channels
Each IT interface channel provides internally isolated
power and communication capabilities to an IT over a
single twisted-pair wire. The module also allows the
use of an external power supply to power the IT.
The use of an external power supply common to two
or more loops requires the use of a Cable Balun
Module to maintain digital communication line
balance. The baud rate is software-selectable with all
channels at the same baud rate. Each module
provides bi-directional digital communications at
4800 baud rate between the IT and the system
redundant Fieldbus or provides bi-directional digital
communications at 600 baud rate while allowing
simultaneous 4 to 20 mA analog signal for use in
other applications (for example, to an emergency
shutdown system)
The module is an IT host, enabling the system to
receive digital messages from the transmitter in
engineering units. Each message is received ten
times per second at 4800 baud and two times per
second at 600 baud and contains:
• Two measured variables (that is, primary
pressure and transmitter temperature) in IEEE
32-bit floating-point
• Security information
• Diagnostics
• Message checking.
This information is available to all elements of the
system.
Since the communications are bi-directional, the
system can display the output, transmitter
temperature (°C and °F), and continuous self
diagnostics. Additionally, the following information can
be displayed or reconfigured from the console or a
Hand-Held Terminal (PSS 2A-1Z3 A):
• Output in engineering units
• Fail-safe state
• Tag number, name and location
• Device name (letterbug)
• Last calibration date
• Two levels of upload/download capabilities.
The IT interface portion of the module executes the
Intelligent Transmitter application program. The
configurable options for this program are Fieldbus
Switching Enable and Fieldbus Switching Time.
Output Channels
FBM44
Each output channel drives an external load and
produces a 0 to 20 mA output. Each output channel
performs the signal conversion required to interface
the electrical output signals to field actuators from the
redundant Fieldbus.
The output channel portion of the module executes
the analog I/O application program. The configurable
options for this program are Fail-Safe Configuration
(Hold/Fallback), Analog Output Fail-Safe Fallback
Data (on a per channel basis), Fieldbus Switching
Enable and Fieldbus Switching Time.
An optional style of termination cable assembly which
is available includes built-in bypass jacks for each
output channel on the front connector of the Fieldbus
Module. Jacks accept a bypass plug from the
I/A Series Output Bypass Station or other external
20 mA source. This option should be considered for
applications where maintaining output is desired
during maintenance operations, for example, when
replacing a failed module.
Common Characteristics
POWER REQUIREMENTS
Input Voltage (Redundant)
26 to 42 V dc (nominal)
Consumption
11 W (maximum)
Heat Dissipation
8 W (maximum)
ISOLATION
FUNCTIONAL SPECIFICATIONS
Transmitter Channels (Cont.)
MAXIMUM DISTANCE (INTERFACE TO
TRANSMITTER)* (SHIELDED TWISTED-PAIR
WIRES, OR AT MINIMUM TWISTED-PAIR WITH
OVERALL SHIELD)
The module can withstand, without damage, a
potential of 600 V ac applied for one minute
between any channel and earth (ground), or
between a given channel and any other channel.
NOTE
This does not imply that these channels are
intended for permanent connection to hazardous
voltage circuits. Connection of these channels to
voltages greater than 30 V ac or 60 V dc violates
electrical safety code requirements and may
expose users to electric shock.
COMMUNICATION
Via a redundant Fieldbus
Transmitter Channels
INTERFACE
4 isolated and independent channels
ERROR CHECKING
CCITT 2 byte CRC
COMMUNICATIONS
Non-redundant, point-to-point, master/slave,
asynchronous, half-duplex at a software-selectable
baud rate of 600 or 4800 baud. All 8 input channels
must be configured at the same baud rate
(600 baud for 4 to 20 mA transmitters or 4800 baud
for digital transmitters).
SPEED
10 messages per second at 4800 baud, or
2 messages per second at 600 baud
610 m (2000 ft) (shielded twisted-pair wires) at
4800 baud, or 1800 m (6000 ft) at 600 baud
MAXIMUM LOOP RESISTANCE
500 Ω (not including the transmitter). The
maximum resistance between the Hand-Held
Terminal (HHT) and the transmitter is 350 Ω.
TRANSMITTER INTERNAL POWER
24 V dc +4%, -2%, source resista
FBM42