Testing a Modbus TCP connection

step 1 install a modbus client you need a tool to connect to and browse your modbus tcp device here are some free and common options client name platforms link modscan / modsim windows download schneider modbus tester windows download eurotherm itools (for eurotherm) windows download each client lets you manually connect to a modbus tcp device, specify a unit id (slave id), function code, register address, and data type, then read or write values step 2 prerequisites before testing, make sure you know the modbus slave address (default ff is broadcast) know the register address and data type you want to read example reading a 32 bit float from register 40001 on slave 01 step 3 test the connection open your chosen modbus client enter the ip address and tcp port (default 502) specify slave id e g 01 function code typically 3 (read holding registers) start address e g 40001 quantity number of registers to read click connect or read to view the result troubleshooting modbus tcp connection 1\ slave id format the slave id in the collector is in hexadecimal notation , not decimal example device id shown as 10 decimal = 0x0a hex use an online converter slave ids must always be two digits for example, 0a instead of a 2\ unknown slave id if the slave id is not known set ff (255) as the broadcast address, all slaves will receive it first slave will answer use ff only if one device is connected never use broadcast (ff) on multi device networks 3\ 1 based vs 0 based addressing the go modbus library used by the collector uses 0 based indexing example if documentation says register 1006 , use address 1005 in the collector configuration 4\ modbus tcp vs modbus rtu the factry modbus collector supports modbus tcp only if you have a modbus rtu (serial) device, use an external tcp to rtu gateway or bridge software example moxa mb3180 or modbuspal emulator