Modbus
Introduction
Links
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Main Landing Page: http://www.modbus.org/tech.php
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Modbus Protocol Spec: http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf
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Modbus TCP Protocol Spec: http://www.modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf