I am developing handshake logic for PLC and Scada (Gateway) as shown in the attachment, 

The logic works good but there is an issue that flag always remains set from scada and even if there is network failure, the output is shown as Q2 (network ok) unless the M40 flag is manually reset.

The scada just sends 1 ie set to flag after every 4 seconds before the timer of PLC logic elapses. 

Please suggest appropriate changes to the logic. Ignore the I1 attached in the logic as it was added for manual simulation, there is no physical input to reset the logic using PLC input. 

Attaching the diagram zip file in next post. 

Here is the actual logic in this attachment. 

All communication is done using Modbus, no S7. 

Thank you for the PLC logic correction. To make it properly work from gateway software, i now send 1 and 0 after every 2 seconds thus resetting the flag instead of just sending 1 as it was done previously..

 This works and requires us to set and reset the flag after every 2 seconds from the gateway..if the flag state is unchanged after 5 seconds, then the communication break output is set to high, else its Communication OK output.  

Thank you 

ixo65

Try this ...