Fix Siemens S5 Handshake Delay from Peripheral troubleshooting

I’m looking for an experienced Siemens S5 specialist who can quickly diagnose and eliminate a delayed handshake problem between the CPU and a digital output module. The PLC is running fine otherwise, but the output stage is taking several extra cycles to acknowledge commands, which is disrupting downstream equipment.

PLC: Siemens S5 – 6ES5 943-7UB21 (CPU 943B)
Two racks connected via IM306 ↔ IM306 (6ES5 306-7LA11)

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Rack 1 (Main Rack)
• 6ES5 951-7LB21 – Power Supply
• 6ES5 943-7UB21 – CPU
• 6ES5 523-3UA11
• 6ES5 460-7LA13
• 6ES5 470-7LC13
• 3 × 6ES5 454-7LA11
• 6ES5 308-3UC11
• IM306 (6ES5 306-7LA11) to Rack 2

Rack 1 operates normally.

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Rack 2 (Expansion Rack)
• IM306 (6ES5 306-7LA11)
• 6ES5 454-7LA11
• 6ES5 451-7LA12
• 5 × 6ES5 430-7LA12 (Digital Output modules)
• 6ES5 420-7LA11

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Reported Problem
• All modules operate correctly except two 6ES5 430-7LA12 digital output cards in Rack 2.
• CPU reports “delayed handshake from peripheral.”
• Outputs on those two modules do not respond.



I am working with IBH S5 for windows