GE 8755-CA-NS EBIM carrier
Mixed I/O types within a single node
IS and non-IS field wiring types can also
be incorporated within one PAC8000 node
(see Figure 5). In this arrangement, the two
parts of the node are separated by a ‘Railbus
Isolator’ module. The Railbus Isolator provides
a section of internal communications bus
(‘Railbus’) for the IS I/O modules which is
protected from invasion by damaging fault
voltages. A single PAC8000 node (under the
command of one Bus Interface Module or
Controller) can then support a mixture of
certified IS field devices, certified Division 2 or
Zone 2 field devices and general purpose I/O,
including AC mains circuits. Only one Railbus
Isolator is used per PAC8000 node.
Related 8000 Literature
INM8000
Installation Guide
GENERAL
All I/O modules are connected to the Bus
Interface Module (BIM) or Controller via a
proprietary bus system called ‘Railbus’ and
one BIM can control up to 64 modules. The
module carrier provides the transmission
medium for the Railbus and, by plugging
a module onto a carrier, connections are
made between the module and the bus. The
connectors on the carrier also provide the
power supply links to the module and, when
required, power for the field wiring.
Addressing of I/O modules
Modules are addressed by the BIM or
Controller in terms of their position, or slot, in
the total chain of 64 modules not by individual
module types. As a result, a module can be
removed and replaced by another of its own
type without the need to ‘tell’ the system of
the change. During configuration, the node
is told the characteristics of each necessary
module position whether or not the module is
present at the time. Consequently, if a module
is removed for service replacement, the properties of the ‘slot’ are still retained.
IMPORTANT MODES
Output failsafe mode
Output modules have the ability to assume a
failsafe state. This can happen for two reasons.
1) A module can be forced in to failsafe state
by issuing a specific command to it.
2) Modules have a configurable “timeout”
parameter. This defines the maximum time
period of communication inactivity. If this
period is exceeded the module adopts a
failsafe state.
The different module types have their own
response to a failsafe command, and those
responses are described in the individual
sections that follow.
Input fail values
In the event of failure of an input module, the
reported value is forced to a predefined state –
low, high or hold last value. This ensures that
the host adopts a state consistent with safe
operation of the plant.
Power-up/initialization state
When powering-up a node it is essential
for plant safety that the state of each of
the outputs is known. While the node is
initializing, the I/O modules are held in the
power-up state (see following pages). After
node initialization and before establishing
external communication, the outputs are set
to predefined “initialization” states. This “safestate” can be defined by the user for each
output channel.
Non-volatile configuration memory
The configuration information for all I/O
modules in a node is stored in non-volatile
memory (NVM). When a module is replaced,
when the node is powered up or following
a reset, the node will download the stored
configuration information to the relevant I/O
modules. Visual indicators LEDs are provided
on each module to indicate Power, Fault and
channel Status information. These are based
on the NAMUR NE44 specification for LED
indicators. The Power and Fault indicators are
common to all I/O modules and their states
are shown in the following tables.

HART® capability
AI modules “with HART” can obtain information from HART instruments of protocol
revision 5.0 or later. Each channel can
communicate with a single HART instrument.
HART universal command 3 is used to gather
up to 4 dynamic variables and status from
each HART instrument. This provides more
process information to the control system
from each device. Greater accuracy can
also be achieved by eliminating A/D and D/A
errors. In addition, HART pass-through may
be used for device configuration, calibration
and advanced diagnostics.
IMPORTANT MODES
Output failsafe mode
The AI modules have eight user-channels that
are sampled every 27 ms (2/2) or 33 ms (2/1).
Data format
The input signal is stored as a 16-bit
unsigned value. In this range 0 is equivalent
to 0mA and 65,535 is equivalent to 25mA.
Any digital HART data is stored in its original
IEEE754 floating point format.
Filtering
The Analog Input modules use a firstorder software filter that provides 12 dB
attenuation at the Nyquist frequency of the
algorithm. The filter supports a set of options
that can be matched with control algorithm
execution rates.
Input alarms
Four configurable alarm levels are provided
for each channel—two high and two low (see
figure below). When an input value exceeds
an alarm limit a flag is set and the BIM gets
a new alarm status.
Alarm deadband
The Alarm Deadband prevents the alarm from
tripping on and off because of system noise.
It can be configured for each channel and is