How to Fix the Allen-Bradley Micro820 Modbus TCP Socket Error

Polling third-party devices via Modbus TCP on an Allen-Bradley Micro820 is often accompanied by a notorious integration roadblock: the Socket Exhaustion Error. All is well for a few hours, then suddenly all MSG_MODBUS or MSG_CIP instruction blocks stop working, data stops updating, and the only temporary fix is to hard-cycle the power to the PLC.

Block: MSG_MODBUS2
Enable:  TRUE
Done:    FALSE
Error:   TRUE
ErrorID: 16#0081 (or 16#000C)

Description: Connection timed out or no available TCP sockets to establish a new connection. The PLC's internal socket pool is exhausted.

This is not a bug in your ladder logic’s register mapping; it is a rigid hardware limitation. Unlike ControlLogix processors which manage massive CIP connections dynamically, the Micro820 has a highly restricted number of concurrent TCP sockets. If connections are not explicitly closed or if they get trapped in a TIME_WAIT state due to network jitter, the PLC literally runs out of sockets to open new Modbus queries.

Step 1: Implement Explicit Socket Closure

By default, if you are rapidly triggering a MSG block to a Modbus TCP device, the PLC attempts to keep the socket alive. However, if the field device drops the connection unexpectedly, the Micro800 processor might keep the socket reserved, waiting for a graceful FIN/ACK packet that never arrives.

Step 2: Stagger Your Polling Sequence

If you have 5 different Modbus TCP devices on the network, do not trigger all 5 MSG blocks simultaneously using a single clock pulse. This will instantly max out the connection limit.

• Build a token-passing state machine. Allow MSG Block #1 to complete and verify its Done bit.
• Once MSG Block #1 is finished, trigger MSG Block #2.
• If a block returns an Error, bypass it immediately rather than letting it hang, and log the failure.

// Start an explicit Timeout Timer for the MSG Block (2 seconds)
MSG_Timeout_TON(IN := MSG_Trigger, PT := T#2s);

// If MSG completes successfully, move to the next device
IF MSG_Block.Done THEN
    MSG_Trigger := FALSE;
    State_Machine := 2; // Pass token to Device 2
END_IF;

// Trap: If Error occurs OR Timeout is reached, forcefully KILL the trigger
// This prevents the Micro820 socket from hanging indefinitely
IF MSG_Block.Error OR MSG_Timeout_TON.Q THEN
    MSG_Trigger := FALSE; 
    Fault_Counter := Fault_Counter + 1;
    State_Machine := 2; // Skip device and move on
END_IF;