How to convert ModbusTCP to ModbusRTU on IR809

Got Cisco IR809 and would like to set modbus gateway to convert ModbusTCP packet to ModbusRTU that direct attached on interface async 0.
MODBUS/RTU(Slave)------------(rs485)----------IR809---------------(ethernet)------------MODBUS/TCP(Master)
Tried to use raw-socket but it look like when the Slave receive message it is MODBUS/TCP message, it shows bad CRC. The Cisco809 have to convert the TCP to RTU. Tried to read the solution of IOx but unable to find the work configuration and code.

Got an application called ES200 that runs on the IOx and can solve the problem. It is CISCO Copmatible and listed on Cisco Marketplace