Rockwell Automation T80004 User Manual

TrustedTM

AN-T80004

Application Note

Field Loop Configuration

The TrustedTM Input/Output (I/O) modules are designed to monitor field loops for alarm and field cable fault states. The configuration of field loops differs between modules.

This application note:

• explains the circuits required to allow line monitoring of field wiring.

• describes connection and configuration options for different device types, including system and

field powered types, volt-free, current sourcing and sinking and two, three and four wire devices.

• suggests options for isolation and protection.

Issue Record

Issue Number

1 Feb 07 Initial Issue

2 Aug 07 T8297 connections

3 Dec 07 Circuit options

4 Feb 08 Comments incorporated

5 Jun 08 De-en LM Outputs

6 Sep 08 System cable design

7 Dec 08 Low current loads

8 Feb 10 2,3,4 wire AI,8472 nlthresh

9 Aug 12 Clarifications to DI line monitoring

10 Jun 13 T8403 states 0 and 6 not used

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Table of Contents

Analogue Inputs ...................................................................................................................................... 5

Line monitoring ................................................................................................................................... 5

T8431 and T8432 Termination Options ............................................................................................. 5

Current sourcing and sinking, active and passive loops ................................................................ 6

Two, three, four wire connections .................................................................................................. 7

Millivolt inputs ................................................................................................................................. 9

T8433 Termination Options .............................................................................................................. 10

Configuration .................................................................................................................................... 13

Threshold online update .............................................................................................................. 14

Overvoltage Faults ........................................................................................................................... 15

Voltage signals to Analogue Input Modules ..................................................................................... 15

Background: Input impedance test .............................................................................................. 15

Workaround ................................................................................................................................. 15

MTEST Diagnostics ..................................................................................................................... 15

Digital Inputs ......................................................................................................................................... 16

Thresholds ........................................................................................................................................ 16

Line Monitoring and Marshalling for Volt-Free Inputs ....................................................................... 17

T8402 Dual Inputs ........................................................................................................................ 17

T8403 TMR Inputs ....................................................................................................................... 18

T8423 35-120V dc Digital Inputs .................................................................................................. 19

T8424 120V ac Digital Input ......................................................................................................... 20

Non volt-free digital inputs ................................................................................................................ 21

Current controlled inputs .............................................................................................................. 21

Field powered inputs .................................................................................................................... 22

Hall Effect and Open Collector/Open Drain ................................................................................. 23

Zone Interface and Valve Monitor Inputs .............................................................................................. 24

Termination Options ......................................................................................................................... 24

Diagnostic Test Termination ............................................................................................................ 24

Configuration .................................................................................................................................... 25

Frequency/Pulse Inputs ........................................................................................................................ 26

Digital Outputs ...................................................................................................................................... 27

Power Groups .................................................................................................................................. 27

Line Monitoring ................................................................................................................................. 28

No field supply voltage (state 1) ................................................................................................... 28

Output de-energised/off (state 2) ................................................................................................. 28

Open circuit in field wiring or load (state 3) .................................................................................. 28

Output energised/on (state 4) ...................................................................................................... 29

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Short circuit in field wiring or load (state 5) .................................................................................. 29

Field fault (state 6) ....................................................................................................................... 29

Channel fault (state 7+) ................................................................................................................ 29

No-load (Open Circuit) Threshold .................................................................................................... 30

Triplicated Modules 8448, 8449, 8451, 8461, 8471 ..................................................................... 30

Quad Structure Modules 8472, 8473 ........................................................................................... 31

Termination ...................................................................................................................................... 32

T8842 VFTA ................................................................................................................................. 32

T8850 and T8870 FTAs ............................................................................................................... 33

Power Termination ....................................................................................................................... 34

T8472 Termination ....................................................................................................................... 34

Terminating Resistors .................................................................................................................. 35

Volt-free outputs ........................................................................................................................... 35

Hazardous area outputs ............................................................................................................... 36

Device Characteristics ...................................................................................................................... 36

Low current loads ......................................................................................................................... 36

Dynamic loads.............................................................................................................................. 36

High inrush current loads ............................................................................................................. 37

Power supply fluctuations ............................................................................................................ 37

Analogue Outputs ................................................................................................................................. 38

Module circuit ................................................................................................................................... 38

Line Monitoring ................................................................................................................................. 38

No field supply voltage (state 1) ................................................................................................... 38

Output de-energised (state 2) ...................................................................................................... 38

Open circuit in field wiring or load (state 3) .................................................................................. 38

Output energised (state 4) ........................................................................................................... 39

Current demand cannot be met (state 6) ..................................................................................... 39

Channel fault (state 7) .................................................................................................................. 39

Termination ...................................................................................................................................... 39

Hazardous area outputs ............................................................................................................... 39

Pulse Outputs ....................................................................................................................................... 40

Other Issues ......................................................................................................................................... 43

System Cable Design ....................................................................................................................... 43

Analogue or Digital Input module to FTA ..................................................................................... 43

Analogue or Digital Input module to VFTA ................................................................................... 44

Digital Output module to FTA, Standard Cable ............................................................................ 45

Digital Output module to FTA, integral power cable .................................................................... 46

Digital Output module to VFTA .................................................................................................... 47

Analogue Outputs ........................................................................................................................ 48

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Floating Earth Systems and Loop Isolation ...................................................................................... 49

Hazardous Area Protection .............................................................................................................. 49

System installation ....................................................................................................................... 49

EEx certification ........................................................................................................................... 49

Non-incendive certification ........................................................................................................... 49

Lightning Protection .......................................................................................................................... 49

HART interfacing .............................................................................................................................. 50

Cable earth and screen wires ........................................................................................................... 51

Free Wire Cables ............................................................................................................................. 52

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Analogue Inputs

Line monitoring

Field loops providing analogue input signals to a TrustedTM system do not require line monitoring components to be fitted to the field device, because the measured current acts as the proof of the presence of the line. However, analogue inputs should be checked for accuracy periodically because partial shorts will offset the measurement.

T8431 and T8432 Termination Options

The T8431 and T8432 TrustedTM Analogue Input modules are designed to monitor input signals in the range 0-6V (0 to 22mA via a 250R resistor). The input zero volt reference wires are linked together across the module, so although there is isolation between field and system, there is no isolation between channels. The T8431 has 40 triplicated input circuits and the T8432 has 60 dual input circuits.

Field termination assemblies are available to provide field terminations for most applications.

The T8830 allows connection of system powered two wire devices to a T8431. It requires an FTA cable, TC-201, 203, 501 or 503, to connect to its cable socket. It does not provide a zero volt reference to field devices, so it is only able to directly connect to system powered devices.

The T8831 is similar but replaces the fuse with a resistor, limiting available power for non-incendive applications (non-incendive hazardous area specifications specify limited loop power and are used in the Americas).

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AN-T80004 Field Loop Configuration

The T8842 Versatile Field Termination Assembly (VFTA) provides individually configurable channel circuits for a T8431 and other low voltage 40 channel modules. It also provides three-wire connection (24V supply, signal and zero volt reference). It requires a VFTA cable TC-211, 212, 511 or 512. Circuit configuration is given in PD-8842. Fuse ratings may be chosen up to a total of 3A per group of eight channels. For analogue inputs, a link is required in position E, a 50mA fuse in position A to limit the current loop, and a 315mA fuse in position B. If the device requires more power, fit separate fused terminals alongside the VFTA.

Field Power TBFPn

Channel 1

Signal TBGn

Precision Resistor

Power Resistor

Signal

Field

Terminals TBn

Four-wire devices also require a power 0V return as well as a signal reference. For power less than 315mA, fit both zero volt wires to terminal 3. If separate fuse terminals are required, also fit separate zero volt terminals.

Other VFTAs have existed but were all fixed versions of the T8842.

Current sourcing and sinking, active and passive loops

The T8431 and T8432 all expect to measure the voltage on the channel with respect to the zero volt reference. In the current loop circuits above, current flows into the system circuit, down through a precision resistor (thus creating a voltage drop), and into the zero volt reference. The input channel voltage above the zero volt reference is therefore a measure of the loop current.

System circuit Passive field loop Active field loop

24V

Channel

In the diagram above, an active or field powered loop drives current round the loop. It may have its own local power supply, or be powered from the system through VFTA or separate terminals.

A passive or system powered loop takes a little power from the system supply to drive a variable loop resistance which controls the loop current. This is the reason that current loops are usually 4 to 20mA; the spare 4mA is used to power the resistance control.

All these devices are current sourcing, that is, they supply current to the system’s channel connection. Some devices are current sinking; they take current from the channel connection. These devices cannot be directly connected to T8431 or T8432 inputs, because the system circuit would have to measure voltage from the supply rail with an upside-down circuit, as shown below. This circuit cannot be used.

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System circuit

24V

Channel

This circuit cannot be used.

Usually the current loop is not even returned to the system but is added to the field device’s own power zero volt reference. Therefore the only available measurement point for the current is the channel connection. The T8431 and T8432 are not designed to measure voltage with respect to the supply rail, and so the circuit above is not possible. The solution is to fit a current isolator device (e.g. MTL5040) which makes it possible to reflect the current in the opposite direction, allowing it to be fed into the channel connection.

System circuit Active field loop Mirroring isolator

24V

Channel

Two, three, four wire connections

The passive field loop shown on the previous page is a two wire device; the 4-20mA loop current is regulated by the device, and flows out on one wire and back on the other. An FTA is adequate for this circuit, and the 50mA fuse protects the input against shorts.

System circuit Passive field loop

24V

Channel

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The current loop allows 4mA to drive the device electronics. However, most field devices need higher power. This must be provided with a separate power connection. This is now a three wire device. Note that the fuses on the FTA should not be replaced with higher rating fuses because a short between the fuse and the channel input may damage the channel. VFTAs include a zero volt terminal, but if FTAs are used, fit a separate zero volt terminal.

Active field loop

Some devices have another connection. These have separate zero volt power and current loop return connections, allowing the current loop to be isolated. For 8000 series analogue inputs, wire the two connections together.

Active field loop

PSU

System circuit

24V

Channel

System circuit

24V

Channel

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Millivolt inputs

The T8840 allows a block of eight channels of a T8431 to be connected to thermocouple, RTD or other millivolt inputs using plug-in converters. It requires a VFTA cable TC-211, 212, 511 or 512, which provides separate plugs for each group of channels. This can be wired via a VFTA T8842 allowing some channels of the module to be used for current loops. Configuration options are given in PD-8840.

The T8841 is similar to the T8840 but is specially designed for RTD inputs.

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T8433 Termination Options

The T8433 TrustedTM Isolated Analogue Input module has individually isolated inputs. These measure 0-22mA. The module has a three-wire connection for supply, reference and signal; the extra wires mean that there is only room for 20 channels. The module input circuit is completely isolated and requires an external supply of 4.5 to 8 volts, derived in the circuit below from the loop.

SUPPLY

REFERENCE

TB1 A

7V5

100R

TB1 B

SIGNAL

The diagram above shows the input circuit using a T8833 FTA, a T8433 isolated analogue input module and a 60 channel cable TC-601, 603, 701 or 703. 60 channel cables are required due to the extra wires needed.

The current loop is measured across the 100R resistor (creating a voltage drop of 0.4 to 2 volts for 4 to 20mA). However, the isolated input circuit also needs a power source of between 4.5 and 8 volts; in the above circuit it is provided by the zener diode using the current in the loop from the transmitter. All circuit permutations need to allow for current measurement and channel power on opposite sides of the common reference wire.

The T8433 FTA above allows for field powered current sourcing inputs, i.e. from devices that have their own power supply and drive the current loop towards the channel. The device needs to have at least 7.5 volts of spare loop drive available.

If the device is passive, i.e. it controls the current flow through itself but does not supply power to the loop, it can be powered from an individual isolated supply as shown below.

TB1 A

SUPPLY

7V5

REFERENCE

100R

+ -

PSU

SIGNAL

TB1 B

This clearly requires an extra stage of terminals between the FTA and the field. Note that although it is possible to use one power supply to power several loops, they would then no longer be isolated from each other.

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The T8834 FTA is a configurable version of the T8433. This also connects to I/O cables TC-601, 603, 701 or 703. It also contains a zener diode for the channel power and a precision resistor for the current

TB3 +

TB2 +

TB3 -

SUPPLY

7V5

TB1 A

TB1 B

100R

4K75

REFERENCE

SIGNAL

TB2 -

to voltage conversion, but it may be wired in several different ways.

For field powered (active) devices, sourcing current to the channel, where the channel current does not go below 4mA:

TB3 +

TB2 +

TB3 -

SUPPLY

TB1 A

TB1 B

TB2 -

7V5

100R

REFERENCE

4K75

SIGNAL

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For field powered devices, sourcing current, where the channel current could go below 4mA (e.g. gas detector optics alarms):

Isolated PSU

7V5

100R

TB3 -

SUPPLY

REFERENCE

4K75

TB3 +

TB2 +

TB1 A

TB1 B

SIGNAL

TB2 -

Since the current loop is on the negative side of the channel power, it is possible to configure the T8834 FTA to power and sense current-sinking devices, where the loop draws current away from the channel. Current sinking devices need a separate power supply.

Isolated PSU

TB3 +

TB3 -

TB2 +

TB1 A

7V5

100R

SUPPLY

REFERENCE

4K75

TB1 B

SIGNAL

TB2 -

Product description PD-T8834 shows the full circuit diagram of a channel. The circuit includes links which could be modified to accommodate other requirements.

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Configuration

There are seven input voltage state thresholds detected by all of the analogue input modules, numbered 0 to 6. States 0 and 6 are ‘out of range’ indications, outside of the normal operating range. The remaining five are configurable using the System Configuration Manager. The voltage transition between each state can be split into a rising transition and a falling transition. In the diagram below, the input goes from state 4 to state 5 as it rises above 2.28V. It returns to state 4 as it falls below 2.24V. There are four configurable pairs of state transitions, numbered T1 to T8 below. These are configured using a threshold template as described in product description PD-8082.

Typical voltage threshold values

Tmax 6.0

T8 2.28

T7 2.24

T6 1.82

T5 1.79

T4 1.52

T3 1.47

T2 1.12

T1 1.11

Tmin -0.5V

The input states can be used to define alarm trips, detector conditions, near-overrange signals, valve positions on analogue position sensors etc. There is room to define states for input conditions like gas detector dirty optics as well as alarm thresholds. Note that state 1 indicates a line fault.

The system configuration for I/O modules can only be loaded when the system is shut down and restarted. This includes threshold changes. However, there is an online method for entering a new set of thresholds using the THRSHIN and THRSHOUT boards which is described below. Alternatively the analogue reading may be converted in the application using comparative logic.

Over-range 6 True

High-High 5

4 or 5

High 4

3 or 4

Normal 3 False

2 or 3

Low 2

Low-Low 1 True

Under-range 0 True

Input Channel State

1 or 2

Line Fault Status

False /

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Threshold online update

The channel thresholds can be read or written using the THRSHOUT and THRSHIN boards in the I/O connection table, through the Toolset Debugger’s online view. The thresholds are shown in raw units, at the same scaling as the AI board (0 = 4mA or 1V, 4096 = 20mA or 5V).

To read a channel’s thresholds, enter the channel number on THRSHOUT channel 3, then change THRSHOUT channel 2 from 0 to 1. The thresholds will appear on the THRSHIN board. In the example below, the thresholds for channel 10 have been requested.

To write new thresholds to a channel, enter the new thresholds on channels 4 to 11 of THRSHOUT. Enter the channel number on channel 3 of THRSHOUT, then change THRSHOUT channel 1 from 0 to

1. In the example below left, new thresholds for channel 13 are been loaded. On also setting channel 2 to 1, the thresholds are confirmed on the THRSHIN board on the right.

The manual processes described above could also be automated through the application.

Having changed the thresholds online, remember to update the system INI configuration template and load it into the system. This will ensure that the changes are loaded on the next black start. The online changes are not saved in the processor’s copy of the INI file.

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Overvoltage Faults

The analogue inputs are not designed to measure voltages up to 24V. The A/D conversion will reach its maximum at about 7 volts. Above this point, the measurement parameters will be at the maximum values. This causes module channel faults (0x52nn) to be reported, which appear as slice faults.

Voltage signals to Analogue Input Modules

This section describes the issues involved with the connection of voltage signals to T8431 analogue input modules without the use of a 250 ohm burden resistor, usually used to convert current loops to voltage signals.

Background: Input impedance test

Current loop analogue signals are wired to a 250 ohm burden resistor to convert 0-20mA to 0-5V (as an example). The input circuits are biased internally to 2.5V. An input impedance test is run periodically inside the module. This test adds a disturbance signal to the input circuit which is later filtered out before the measurement is delivered to the processor. The test measures the effect of the disturbance and calculates the impedance of the input circuit. The 250 ohm burden resistor is factored out of the calculation, to leave the true impedance of the line. Out of range (short circuit or open circuit) impedances are recorded. If an out-of-range impedance appears on several subsequent tests on at least two input circuit slices, then a fault state is declared on the channel.

With a voltage input which has very low or high impedance, the above test will declare a fault after confirmation and voting, which can take a few minutes. The result is that the input state number is set to the fault state and the input measurement is set to -2048 as the failsafe value.

Workaround

The recommended method to enable the use of voltage inputs is to fit a 250 ohm resistor. The value is nominal in this case (e.g. 200-300 ohms) and simply biases the impedance into the accepted test range. For low impedance inputs, the resistor should be wired in series to add impedance to the line. For high impedance inputs, the resistor should be wired in parallel as in a current loop circuit. In both cases, the impedance test should be able to detect line faults.

Correcting the input impedance will also improve the input measurement accuracy at the high and low ends of scale. An offset will be observed at the ends of the range if the input impedance differs significantly from 250 ohms.

MTEST Diagnostics

Applications have been reported that inhibit the module diagnostic tests by loading an ‘Additional CLI’ template with ‘MTEST=OFF’. This is not a recommended workaround. This command will turn off other diagnostic functions in the module in addition to the impedance test. It is not stored as part of the INI configuration in the module. This means that although it will be downloaded from the processor on startup or on offline insertion of a module (where the active module is removed and a new module is inserted), it is not transferred on a hot swap to a module in the secondary module position. Therefore if a hot swap is performed to the secondary position, and the secondary module is left in operation, the impedance test will shut down the inputs after a few minutes.

In these applications, it is necessary to swap back to the primary slot before the diagnostics have run, or to remove and reinsert the module in the primary position. Both of these actions will reload the INI from the processor in full. Removal and reinsertion requires redundancy logic in the application to avoid loss of signal.

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Digital Inputs

Thresholds

Digital inputs are essentially analogue inputs which are interpreted as digital. The voltage on the input circuit is measured and allocated to an input state using thresholds, similar to analogue inputs above. For digital inputs, the state bands are essential, since they define the digital input ‘off/open’ and ‘on/closed’ as well as line fault status. The thresholds are similar to analogue inputs except that they operate over a wider voltage range. There are five configurable states separated by pairs of thresholds for rising and falling state change. There are no THRSHIN and THRSHOUT boards to change the thresholds online.

Typical voltage threshold values

Over-range 7 False True Tmax 36.00

T8 22.00

T7 21.00

T6 12.75

T5 12.25

T4 11.75

Short Circuit (approx 24V) 5

4 or 5 False / False / True True

Contact Closed (approx 16V) 4 True False

3 or 4 False / False / True True

Contact Indeterminate 3 False True

2 or 3 False /

Input Channel State

DI Status

False True

Line Fault Status

T3 11.25

T2 3.00

T1 2.00

Tmin -8.00

Thresholds are defined using a threshold template in the system configuration. Product description PD8082 describes how to edit threshold templates. The example above shows ‘Closed’ defined at nominally 16 volts and ‘Open’ at nominally 8 volts, for a 24V digital input module. These are spaced equally through the input voltage range, which allows broad voltage bands for each state.

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Contact Open (approx 8V) 2 False False

1 or 2 False / True

Open Circuit (approx 0V) 1 False True

Under-range 7 False True

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Line Monitoring and Marshalling for Volt-Free Inputs

Field loops providing digital input signals to a TrustedTM system need line monitoring components if they are safety related inputs or require assurance of operation.

Defining the open and closed states nominally at 16 and 8 volts (as above) simplifies the selection of line monitoring components.

There are two types of 24V dc digital input module; T8402 (dual) and T8403 (TMR). These use different field termination assemblies, which affects the selection of line monitoring components.

Using both the circuits shown below, an open switch on a healthy line will signal 8V to the module, and a closed switch will signal 16V. Thresholds between each state should be placed to allow at least 2 volts error, an indeterminate state between open and closed, detection of open and short circuit and a gap between each of the rising and falling thresholds to prevent rapid cycling between states.

Other configurations may be used, but the thresholds should be set around the expected voltage levels in each state.

T8402 Dual Inputs

The diagram below shows the recommended configuration of a volt-free contact field loop monitored by a TrustedTM Dual 24V dc Digital Input Module T8402, connected through a T8802 FTA. This requires a 60 channel cable, TC-601, 603, 701 or 703. The T8402 has two input circuits for each of the sixty channels.

Field devices forming inputs to T8402 input modules may be conditioned using resistors. This is because the equivalent input impedance is located on the Field Termination Assembly (FTA), therefore when the input modules are paired in a hot swap arrangement, the effective impedance seen by the field loop remains unchanged.

It is also possible to use the zener circuit shown below with T8402 module inputs using a T8802 FTA.

If free wire cables (TC-602, 604, 702, 704) are used to wire to terminals, the resistor must be wired in the terminals. 5K is a nominal value and the more standard 4K7 could be used.

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