# Modbus RTU Master Not Working on RS485 (X20CP0484) and Modbus Card (X20CS1030) **URL:** \ **Category:** B&R Software & Runtime\ **Tags:** fieldbus, modbus\ **Created:** [June 20, 2026, 4:57am UTC](https://community.br-automation.com/t/modbus-rtu-master-not-working-on-rs485-x20cp0484-and-modbus-card-x20cs1030/11662 "2026-06-20T04:57:35Z")\ **Posts on this page:** 1\ **Showing post:** 7
**Author:** ![alexander.hefner](https://dub1.discourse-cdn.com/brcommunity/user_avatar/community.br-automation.com/alexander.hefner/32/111_2.png) [@alexander.hefner](https://community.br-automation.com/u/alexander.hefner)\ **Post date:** [June 20, 2026, 7:38am UTC](https://community.br-automation.com/t/modbus-rtu-master-not-working-on-rs485-x20cp0484-and-modbus-card-x20cs1030/11662/7 "2026-06-20T07:38:25Z")
Now, your MBMopen instance is in a different [error state 20242](https://help.br-automation.com/#/en/6/diagnostics_support%2Fsys_error%2Ffiles%2Fsyserror_br2000_20242.htm). As long as MBMopen returns a status unequal 0, no data exchange can be executed anyway. Error 20242 means that some event variable definition in your data module is wrong, for example a variable is used which is not declared or installed in the PLC. ![image](https://europe1.discourse-cdn.com/brcommunity/original/2X/9/9958784e7325dd37e22bb783e6a48571f6b2a49f.png) Please note that variables are only installed in the PLC, if the’re used somewhere in the task code → if the’re just declared but nowhere referenced in any executing code file, the variables do not exist in the PLC → a data module is not a executing code file. So to ensure that every declared variable is installed inside the PLC, you can a.) check the compiler warnings for “unused variables”, and b.) just call the variables once in INIT UP. Some more information about the data module and devent variables can be found in this thread: > [@Program for Modbus RS485](https://community.br-automation.com/t/program-for-modbus-rs485/6870/6): > > EventPV is a PLC variable, that defines whether the line is executed or not. If left empty the line will be always executed. LocalPV is the variable where the value to write or read is located. In you example the first line writes 3 registers to station number 1 with the value from variable LocalPV4 to offset 0 (HoldingRegister 0) using Modbus Function 16(Preset multiple registers) when EventPV4 is set to true. The second line reads 3 holding registers from station number 1 starting from offs… --- _[View the full topic](https://community.br-automation.com/t/modbus-rtu-master-not-working-on-rs485-x20cp0484-and-modbus-card-x20cs1030/11662)._