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Technical Notes
Redundant Application of Modbus TCP/IP VIM2 Firmware (IOD-4111) Defect Correction
By Nobin William
Product: IOD-4111 - Modbus TCP IP Driver for DeltaV VIM2

Affected Users

Anyone using IOD-4111 (Modbus TCP/IP VIM2 firmware) v4.4.0 through v4.5.1 to communicate with Quantum PLCs using hot standby chassis architecture.

Users with simplex devices, or other redundancy architectures (i.e., non Quantum hot standby chassis) are not impacted.

Affected Firmware Versions

IOD-4111 (Modbus TCP/IP VIM2 firmware), v4.4.0 to v4.5.1

Corrected Firmware Version

IOD-4111 (Modbus TCP/IP VIM2 firmware) v4.5.2

Problem Description

Quantum redundancy with hot standby chassis employs switching IP addresses where the active PLC chassis always has the same Primary IP address and the standby PLC chassis always has the same Secondary IP address. On chassis switchover, the IP addresses also switch, and the new active PLC chassis assumes the primary IP address. Consequently, communications with the VIMs always use the same Primary IP address.

In this architecture, a Quantum PLC is usually configured as a redundant device with “Redundancy with Switching IP” in VIMNet Explorer, as shown below for example:

This functionality is defective in the affected firmware versions.

When “Redundancy with Switching IP” is configured, the active VIM is expected to always communicate with the PLC active chassis Primary IP address. MYNAH Technologies has confirmed this defect where the active VIM is incorrectly sending read/ write commands to the Secondary IP address (which corresponds to the inactive chassis). The Secondary IP responds to the read/write requests normally and so no issue will be detected in DeltaV. When a write message is sent, the Secondary IP will acknowledge the write but will not update the provided register, resulting in “silent” write failures in DeltaV.

Workaround for affected firmware versions

We have corrected this issue in a new version of the firmware. More information is available within the ModbusTCP VIM2 4.5.2 Release Notes.

If upgrading the firmware is not an option, the following procedure must be used to implement the workaround:

1. Open the VIO configuration file in the VIMNet Explorer

2. Locate redundant Quantum devices in VIMNet Explorer that use Switching IP. Right-click the redundant Quantum device and select Properties. The Device Properties dialog will be displayed:

3. Click the Edit button. You should see the following:

4. Click “Simplex Device” instead of “Redundancy with Switching IP”

5. Click OK in this window and OK once again on the Device Properties window to confirm the changes.

6. Save the configuration file with a different name to create an updated VIO file.

7. Upload the configuration to the standby VIM (VIM B was standby in this example)

8. On standby VIM upload completion, open DeltaV Diagnostics to verify normal communications. Then browse to VIM Card 57.

9. Right-click Card 57 and select “Redundant Switchover” and confirm the operation

10. Upload to configuration to the new standby VIM A back in VIMNet Explorer

11. Verify that you have working communication to the PLCs.

Impact and Explanation of Workaround

Active communication: Upload the configuration to the secondary VIM first then force a switchover and upload the configuration to the new secondary VIM to avoid a loss of active communication. There should be no impact in the active communication.

Impact in DeltaV configuration: There will be no impact - this change is only in VIMNet Explorer.

Impact on VIM / PLC communication: This should result in the same behavior as “Redundancy with Switching IP”. The concept of Redundancy with Switching IP is that the active Quantum PLC chassis will always have the Primary IP address and the standby chassis will always have the Secondary IP address. Therefore, the active VIM should only ever be concerned with sending commands to the Primary IP address. On the other hand, the standby VIM "pings" the Secondary IP address to ensure path availability. When either VIM A or VIM B is active, it should only ever communicate to the Primary IP address. There is no scenario where the VIMs should be communicating to the Secondary IP address for process data.

If the VIMs are only ever concerned with a single IP address and because the Quantum PLCs switch their IP addresses so quickly, from the VIMs’ perspective a device that uses switching IP redundancy is effectively a simplex device. The only difference is that there is no ping to the standby chassis from the standby VIM. In all other respects, the result is that the VIMs continue to communicate to the active PLC chassis correctly and there is no negative impact on the VIM/PLC communication.

Corrective Action

MYNAH has corrected the firmware to restore the "Redundant with Switching IP" functionality. This firmware, v4.5.2, has been released and is available for download from user accounts in MYNAH website.

Please contact us for any questions about this workaround.