You've done a plug and play, but there is nothing plug-and-play about Modbus RTU (over RS-485).  Modbus implementation is details and more details from documentation, configuration and more configuration.

The Modbus RTU message is formatted with a Unit ID, a Modbus Function Code, a start address register, a register count and a CRC error check and has very specific and critical timing associated with the end-of-message.  All that to say, does a PTP card generate a correctly formatted Modbus message?  Given the amount of queries on Modbus RTU, I'm not sure.  (I'm a generic Modbus guy and Siemens instrument guy, with no Siemens PLC hardware exposure.)  

Since your problem is Modbus RTU and this is the instrument conference/category, I would suggest that you post in the Industrial Communications category > Communication/Networks with exact information on which PLC and which PTP card you are using and see if one of the Modbus guys can help.