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Kepware Knowledge Base: Solution


Communication Diagnostics for a Channel Using a Modbus Driver Shows TX But No RX


Last Update: 2018/11/07

Problem:
When using Kepware's Modbus drivers, the Communication Diagnostics show TX (transmit) but no RX (receive).

Solution:
1. If using an Ethernet-to-serial converter, verify that it is set for Modbus/TCP-to-Modbus-RTU conversion. If it is, use the Modbus Ethernet Driver. Certain devices that are not supported by the Ethernet driver can use Modbus Serial Driver instead with "Use Modbus TCP framing" option enabled.

2. The device may not respond when a Modbus RTU request is sent to a Modbus ASCII device (or vice versa). Users that want to transfer ASCII strings do not have to use the Modbus ASCII Driver: strings can be transferred over Modbus RTU. To verify the type of serial Modbus is supported, refer to the device's documentation.

3. When sending a Modbus RTU or Modbus ASCII request to unit address 0, the device is not supposed to respond; however, native Modbus/TCP devices will normally respond to requests sent to unit address 0.

Note:
Most suggestions that apply to the Modbus RTU protocol (used by the Modbus Serial Driver) also apply to the Modbus Ethernet Driver when connected to a Modbus/TCP-to-Modbus-RTU Ethernet-to-serial gateway. This is because the device on the serial side of the gateway receives the requests in Modbus RTU form.

Related Protocols
Modbus Ethernet