- flowmeter's RS-485 port supports Profibus DP, not Modbus RTU
- Modbus RTU is optional and the option was not purchased, so the flow meter does not actually have Modbus RTU functionality.
- Modbus RTU functionality is resident on the flowmeter, but has not been enabled
- Modbus port misconfigured for Modbus ASCII or Modbus Enron/Daniels, not Modbus RTU
- wiring connector is RJ-45 and the tech just plugged in a standard CAT cable and hopes the wiring pinout is correct, but it isn't.
- miswiring somewhere along the bus
- connection to the wrong physical serial port
- damaged wiring, crushed cable, disconnection in JB
- cable is not twisted pair
- cable is not on the order of 120 Ohm impedance range
- a really long stub cable from the multidrop to the flow meter
- lack of signal ground connection
- use of the shield/drain in the cable as a driver line or the signal ground
- grounding the shield/drain at multiple locations inducing ground loops
- lack of biasing
- lack of terminating resistor
- terminating resistors used in the middle of multidrop runs, not at the ends
- driver lines A and B reversed; won't damage the drivers but it won't communicate
- one end is 4-wire RS-485 and the wiring is not jumpered correctly for 2-wire RS-485
- proprietary requirement to use an ID or PIN number to access the Modbus RTU functionality
- electrical noise is so excessive it creates false bits that corrupt all communications
- ground loop
- too much common mode voltage between signal grounds saturates the UART
- wrong serial settings
- attempted correction of serial settings did not include power cycling the device, so the change has not been recognized.
- wrong slave node ID being addressed
- multiple slave/servers at the same slave node ID number
- RS-485 port misconfigured as RS-232 port
- excessive loads/number of devices on a multidrop bus.
- 2nd master on an RTU bus disrupts communications
- exceeding the cable length limitation for the baud rate
- flow meter responds too quickly, turn-around is too short
- damaged RS-485 port on either end
- flow meter not powered
- excessive latency when a radio link is involved
- a failed slave's RS-485 port on a multidrop can bring down the entire bus
- proprietary requirement to use an ID code or PIN number to access Modbus RTU functionality
