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Implementing Redundant Analog Input Channels with Pcs7AnIn in SIMATIC PCS 7 V9.0

Implementing Redundant Analog Input Channels with Pcs7AnIn in SIMATIC PCS 7 V9.0

In SIMATIC PCS 7 V9.0, when using hardware-redundant analog input modules (such as redundant ET 200M or ET 200SP HA AI cards), you instantiate only ONE 'Pcs7AnIn' channel driver block in CFC for each redundantly acquired signal. Connect the 'PV_In' pin of 'Pcs7AnIn' exclusively to the symbolic address of the channel with the lower (least significant) I/O address. The PCS 7 Module Driver Generator automatically configures the underlying diagnostic blocks, handles channel passivation, and performs bumpless switchover between the redundant modules transparently.

Table of Contents

Executive Summary

Configuring high-availability field instrumentation in SIMATIC PCS 7 V9.0 and V9.1 requires distinguishing between two architectural paradigms: hardware-redundant I/O modules and technological sensor redundancy. A recurring engineering question during CFC development is whether a redundant pair of analog input modules requires two separate Pcs7AnIn (FB1869) channel driver blocks or a single block. This article explains the PCS 7 Module Driver Generator's 'Lowest Address Rule', how the Basis Library automatically constructs @ diagnostic driver structures for redundant I/O, and when multi-channel voting blocks (RedAn) must be applied.

Architecture and Fundamental Mechanics

Architectural Mechanics of Redundant Analog Signal Acquisition:

  • Hardware-Redundant I/O vs Sensor Redundancy:
  • Hardware-Redundant I/O: A single physical field transmitter is wired to a redundant pair of I/O modules installed on active bus modules (ET 200M) or redundant terminal blocks (ET 200SP HA). The automation system provides hardware fault tolerance against module failure, power failure, or bus interface dropouts.

  • Sensor Redundancy: Two or three distinct physical transmitters measure the same process variable and wire into separate, non-paired I/O channels. This protects against field instrument failure.

  • The 'Lowest Address Rule' in CFC:

  • In hardware-redundant I/O configurations defined in STEP 7 HW Config, the PCS 7 Module Driver Generator couples the redundant channel pairs automatically.
  • Only one Pcs7AnIn (FB1869) channel driver block must be instantiated in CFC per redundantly acquired signal.
  • The input pin PV_In must be interconnected directly to the symbol corresponding to the channel with the lower (least significant) peripheral input address (e.g. if the primary channel is IW 512 and the redundant partner channel is IW 544, connect PV_In to the symbol for IW 512).

  • The CFC compiler scans HW Config, detects that IW 512 is paired with IW 544, and automatically configures the underlying @ system diagnostic blocks (CH_AI, MOD_D1, RACK) to monitor both modules.

  • Runtime Failover and Passivation Behavior:

  • Under normal operation, the driver reads from the active primary module. Signal status PV_Out.ST reflects 16#80 (Good).
  • If the primary module fails, passivates, or suffers a wire break, the system driver performs an immediate bumpless switchover to the redundant partner channel without altering the Pcs7AnIn output value.

  • If both redundant channels fail, the driver sets PV_Out.ST = 16#00 (Bad) and asserts the fault flag Bad = TRUE.

  • Contrast with Sensor Redundancy:

  • When two independent field transmitters are used, two separate Pcs7AnIn blocks must be instantiated. Their outputs are then fed into a voting or selection block such as RedAn (FB1854) from the PCS 7 Redundant Block Library, which evaluates signal discrepancies and selects average, median, or lowest value.

Step-by-Step Implementation Procedure

Step-by-Step Procedure to Configure Redundant AI Channels in PCS 7 V9.0:

  1. Configure Module Redundancy in HW Config:
    - Open HW Config in SIMATIC Manager.
    - Place the distributed I/O station (e.g. ET 200SP HA or ET 200M on redundant PROFINET / PROFIBUS).
    - Insert the first Analog Input module into Slot N.
    - Insert the second identical Analog Input module into the paired partner slot (e.g. Slot N+1).
    - Open the module properties of the first module -> select the Redundancy tab.
    - Enable Redundant operation and designate the partner module slot.
    - Configure discrepancy time and tolerance parameters.
  2. Define Symbols in the Symbol Table:
    - Open the Symbol Table in SIMATIC Manager.
    - Assign a symbolic name to the lower address channel (e.g. PIW 512 -> TT_101_ACT).
    - Assign a distinct symbol to the partner channel (e.g. PIW 544 -> TT_101_RED).
  3. Instantiate Pcs7AnIn in CFC:
    - Open the target CFC chart.
    - Drag one instance of Pcs7AnIn (FB1869) from the APL library into the chart.
    - Connect input pin PV_In directly to TT_101_ACT (the lower address).
    - Do not connect TT_101_RED to another Pcs7AnIn block.
  4. Compile CFC Charts with Module Driver Generation:
    - Select Chart -> Compile -> Charts as Program.
    - Ensure the checkbox Generate module drivers is enabled.
    - Execute compilation. Verify that the compiler automatically generates the diagnostic links to both modules in the @ system charts.
  5. Download to AS and Test Failover:
    - Download the user program to the CPU 410 / S7-400.

Verification

Verification Procedure:
1. Online Signal Verification: In CFC Test Mode, open Pcs7AnIn. Verify that PV_Out.Value matches the field transmitter reading and PV_Out.ST indicates 16#80 (Good).
2. System Chart Verification: Open the generated system chart @(1) or @(2). Verify that the auto-generated diagnostic driver block monitors both primary and secondary module addresses.
3. Physical Failover Test: Disconnect the primary analog module or pull its front connector. Verify that Pcs7AnIn output remains steady without dropping to 0, an operator system alarm reports module passivation, and PV_Out.ST maintains valid operation.

Limitations and Operational Precautions

Operational Precautions and Architectural Constraints:
- Never connect both addresses of a redundant module pair to separate Pcs7AnIn blocks. Doing so causes driver generator conflicts, duplicate diagnostic reporting, and potential system driver errors.
- Discrepancy Window Tuning: Ensure the discrepancy time configured in HW Config is long enough to absorb module conversion lag but short enough to protect process dynamics.
- Redundant sensor voting (e.g. 2oo3 voting) is not handled by single Pcs7AnIn driver blocks; it requires multiple channel blocks feeding technological blocks such as RedAn.

Engineer FAQ

Do I need two Pcs7AnIn blocks for redundant AI cards in PCS 7 V9.0?

No. Exactly one Pcs7AnIn block is used for each redundantly acquired channel pair. The module driver generator manages both physical modules transparently.

Which address should I connect to Pcs7AnIn in a redundant module pair?

Always connect PV_In to the symbolic name of the channel with the lower (least significant) peripheral address.

What happens if the primary redundant AI module fails?

The underlying PCS 7 system driver detects module passivation and performs an immediate, bumpless switchover to the partner module without disturbing the channel output.

When should I use the 'RedAn' block?

Use RedAn (FB1854) when you have two distinct physical transmitters measuring the same process variable (sensor redundancy), requiring 2 separate Pcs7AnIn blocks feeding into RedAn for voting.

Does module driver generation support ET 200SP HA redundant AI modules in PCS 7 V9.0?

Yes. PCS 7 V9.0 natively supports ET 200SP HA redundant terminal carrier assemblies with automated module driver generation.

Technical References

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Related Tags

  • Pcs7AnIn
  • Hardware Redundancy
  • Module Drivers
  • ET 200SP HA
  • Analog Inputs
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