Lantronix XPort APS: Modbus Protocol User Guide - Page 14

3: Configuring Modbus Delay after CTS Going Active (0-1275 ms, 5ms increments) Only asked if RTS/CTS mode is variable and set to wait for CTS to go active. After the IAP Device Server sees the modem assert an RTS/CTS response input, it delays from 0 to 1275 msec before transmitting. If the IAP Device Server waits without seeing a valid response from the modem, it will return the Modbus exception response 0x0B (hex) to the Modbus/TCP requesting master. Delay Dropping RTS after Transmitting (0-1275 ms, 5 ms increments) Only asked if RTS/CTS mode is variable. After the IAP Device Server completes transmission, it delays from 0 to 1275 msec before dropping the RTS/CTS output. Advanced Modbus Protocol Settings Changing these parameters takes a bit of thought and planning. Slave Address (0 for auto, or 1...255 fixed otherwise) Modbus/TCP includes a Unit ID field, which is used to address multiple Modbus slaves at a single IP address. Unfortunately, some first generation software drivers assumed a single slave at each IP and always set the Unit ID field to 0. This causes the IAP Device Server problems because it requires the Unit ID for the Modbus/RTU "Slave Address." To support these older applications, the IAP Device Server allows you to force a fixed address for Modbus/RTU and Modbus/ASCII, but note that this restricts you to a single serial slave device per IAP Device Server. Setting this value to 0 causes the IAP Device Server to use the Modbus/TCP Unit ID as received. Setting it to any other address causes the IAP Device Server to always use the set value as a fixed address. Allow Modbus Broadcasts (1=Yes 2=No) This relates to the previous issue. The default is 2/No, in which case the IAP Device Server always assumes a Modbus/TCP "Unit ID" of 0 really means Modbus slave address 1. Answering No here is like setting a fixed address of 1 (parameter above), except the fixed address is only used if the Modbus/TCP "Unit ID" is 0. Note: In the current software version for IAP Device Server, a true Modbus broadcast is only supported when a serial slave device is attached. A Modbus broadcast from a serial master device is discarded regardless of this parameter setting. Use MB/TCP 0x0B/0x0A Exception Responses (1=No 2=Yes) Traditional serial Modbus uses silence to signal some errors. While this works well with direct serial lines, it causes serious problems on a TCP/IP wide-area-network where delays are not so predictable. . See 2: Modbus for more information. Setting this parameter to 1/No causes the IAP Device Server to behave like a traditional Modbus serial slave - it answers timeouts, unconfigured slave addresses, and CRC errors with silence. Setting this to 2/Yes causes the IAP Device Server to return 1 of 2 new exception codes defined in Modbus/TCP. Consider exception hex 0A (PATH UNAVAILABLE) a "hard" error where a retry is not likely to succeed. It is returned: Modbus Protocol User Guide 14