Error 1 Occured at TCP Write/Read in LabVIEW

There are a few potential causes for receiving Error 1 at TCP Write/Read which are: 

1. Invalid connection ID.
   1. Most common cause: wiring Listener ID instead of Connection ID to TCP Read or Write VIs.
2. Connection was closed or never established.
   1. If the connection was closed earlier or never successfully opened, the refnum becomes invalid.
3. Scope issues.
   1. LabVIEW cleans up refnums when the hierarchy that created them goes idle (e.g., dynamically called VI stops).
4. Multiple opens on the same port.
   1. Attempting to open the same port in multiple places simultaneously can cause invalid references. 

To resolve this issue, work through the following checks:

• Clarify wire types and refnums.
  • Double check settings for Listener ID and Connection ID as they are different:
    1. Listener ID: Returned by TCP Listen.vi, used only for accepting connections.
    2. Connection ID: Returned by TCP Wait on Listener.vi or TCP Open Connection.vi, used for read/write operations.
• Validate the Connection ID before use.
  • Use probes or indicators to confirm that the Connection ID is valid before calling TCP Read.
• Ensure a proper connection lifecycle.
  • Open → Read/Write → Close.
  • Avoid closing the connection prematurely or multiple times.
• Check for scope issues.
  • Keep the VI that opened the connection running while using the refnum. Avoid passing refnums through global variables without proper handling.
• Debug with examples.
  
  • Users can utilize a LabVIEW example as a reference for development and debugging.
  • A recommended example for this use case would be - "Simple TCP.lvproj" found in the LabVIEW example finder under - Networking >> TCP & UDP >> Simple TCP.lvproj
• Handle multiple clients correctly.
  • If using multiple clients, ensure each accepted connection gets its own Connection ID and is properly closed when done.
• Check timeout and Read settings.
  • For TCP Read, ensure bytes to read > 0 and adjust timeout appropriately. Consider using CRLF mode or fixed-size messages for predictable reads.