We (SMAR) make a device that converts FoundationT Fieldbus to Modbus. If the PLC5 can accept Modbus this is one solution.
For integrating fieldbus and industrial Ethernet with legacy system take a look at chapter 5 of the book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" (buy online in hardcopy or download immediately
in softcopy): http://www.isa.org/fieldbuses
If you can't buy the book now, you can download chapter 1 (overview) for free in softcopy form. It's free, but you must register an account. If your email does not support this hyperlink feature correctly, please copy the entire link and paste it into your Internet browser. Mind the line wrap, make sure to get the complete path all the way to the 4585: http://www.isa.org/Template.cfm?Section=Shop_ISA&template=/Ecommerce/Product
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