Lantronix XPort APS: Modbus Protocol User Guide - Page 15

3: Configuring Modbus  If slave-attached - currently never. However, future firmware may allow the user to define the range of valid slave addresses.  If master-attached - if a Modbus request has a slave address that is not configured in the Unit ID to IP mapping table.  If master-attached - if the TCP socket failed to open. This is really a soft-hard error, as the reason the TCP socket failed to open may be transient or a hard configuration error. Consider exception hex 0B (TARGET DEVICE FAILED TO RESPOND) a "soft" error where a retry may succeed. It is returned:  If slave-attached - if the slave didn't answer or the answer contained a CRC error  If master-attached - if a TCP socket is open, but no response was received in the defined message timeout.  If master-attached - if a TCP socket is open, but the remote Modbus/TCP slave/server returned exception 0x0B. Disable Modbus/TCP pipeline (1=No 2=Yes) While the Modbus/TCP standard specification requires Modbus/TCP masters/clients to only issue one poll at a time, the full-duplex flow-controlled nature of TCP/IP allows them to issue more than one at a time, and the TCP socket will buffer them. The IAP Device Server will fetch them one at a time and answer each in turn. See 4: Monitor Mode and Firmware for more information. Setting this to 1/No causes the IAP Device Server to allow this queuing or pipeline behavior. This is the safest default setting - only change this to disable if you are having problems. Setting this to 2/Yes causes the IAP Device Server to always fetch the newest request from the TCP buffer - all older requests are discarded. This allows a Modbus/TCP master/client to retry old requests without risking building up a stale queue of waiting requests. Character Timeout (10-7050 msec) This sets the timeout between characters received. Official Modbus/RTU defines a 3.5 character time-out, but complex devices have various interrupts that can cause 5 to 10 character "pauses" during transmission. A safe value for general use with Modbus is 50 msec. Note: Setting this value lower than 50 msec will not improve performance and may even make performance worse. The IAP Device Server uses an intelligent length-predicting algorithm to detect end-of-message in Modbus/RTU. This "character timeout" is only used with user-defined or non-standard Modbus functions. Message Timeout (200-65000 msec) This sets the timeout for a response from a connected slave both serially and by TCP/IP.*** Serial TX delay after RX (0-1275 msec) (0) This feature inserts a delay between the Modbus/TCP master requests. The first request is sent out of the serial port of the IAP Device Server to the Modbus slave. When the slave's response enters the serial port of the IAP Device Server, it triggers this timer. After the specified delay is reached, the next master request is allowed to pass through the serial port of the IAP Device Server, and the timer is reset. This feature is particularly useful when using RS485 2-wired serial protocol. The delay gives ample time for the RS485 slave devices to turn their transmitters off and their receivers back on. Modbus Protocol User Guide 15