What causes "Exception Error 91" when using ProWorx32 and Modbus?

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Goals and Symptoms

The purpose of this Resolution is to provide the customer with a document to address Modbus connectivity issues
using ProWORX32.

Facts and Changes

The following table shows causes and actions to take to alleviate Exception Error 91 in ProWORX32.

Causes and Fixes

Cause or Symptom	Action
1. Com port is attached by another program.	Disable or uninstall program attached to com port. Common programs that attach com ports are RSLinx, and HotSync Manager
2. Communication Parameters are incorrect.	Default com parameters are: Address - 1 Protocol - Modbus I Mode - RTU Data Bits - 8 Parity - Even Stop Bits - 1 Baud - 9600
3. Windows port settings need to be adjusted.	Open Device Manager Click on Ports Click on Com1 Click on Properties Click on Advanced Verify the "transmit and receive buffers" are set to maximum. Verify the FIFO or XON/XOFF box has a check in it.
4. Cable Pinout is incorrect or has a broken wire. Desktop computers programming 9-pin Modbus RS232 port PLCs.	Quantum Programming Cable 990NAA26320 - 12 ft Quantum Programming Cable 990NAA26350 - 50 ft. Compact Programming Cable AS-W952-12 - 12 ft Momentum with 9 pin Modbus connectors can use any of the above.
5. Cable Pinout is incorrect. Laptop computers programming 9-pin Modbus RS232 port PLC.	Alternate jumpered pinout. Note DTR and DSR are jumpered out. This ties handshaking high so a laptop com port with lower voltage can connect to our PLCs.
6. Cable pinout is incorrect or has a broken wire. Desktop computers with RJ45 Modbus RS232 PLC ports. PLCs with RJ45 Modbus programming ports are Compact, Momentum, and the Modicon Micro.	You need to purchase a prewired kit adapter to build this pinout. RJ45 to 9-pin Adapter 110XCA20300 RJ45 Programming Cable 110XCA2820x where x = 1, 3 ft 2, 10 ft 3, 20 ft
7. Cable pinout is incorrect. Laptop computers with RJ45 Modbus RS232 PLC ports. PLCs with RJ45 Modbus programming ports are Compact, Momentum, and the Modicon Micro.	See programming cable part numbers in previous step. RJ45 to 9-pin Adapter - 110XCA20301 The adapter kit for this configuration is NOT prewired. Alternate jumpered pinout. Note DTR and DSR are jumpered out. This ties handshaking high so a laptop com port with lower voltage can connect to our PLCs.