GE Analog Input Modules

GENERAL

The 4–20 mA AI modules provide digitized

data and status information from 4–20 mA

current loop sensors.

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

always set on the ‘inner’ side of the alarm limit

to be, typically, greater than the system noise

in the plant. If an alarm is activated, it will

remain until the input moves the full extent of

the deadband towards a “safer” value.

The Hi-Hi and Lo-Lo alarms support the

NAMUR recommendations, i.e. if the alarm

limit is set less than 3.6 mA (Lo-Lo), or greate

I/O Modules

Overview

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