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1. Genesis Controller Modbus Communication Protocols
The Genesis Controller heat trace controller is equipped with Ethernet ports, and can be
readily connected to a distributed control system (DCS). The controller may be networked
to a host PC, running windows-based Genesis Network client-server software, for
remotely programming or monitoring and alarm status. The Genesis Controller supports
the most widely used industrial Modbus protocol with standard Ethernet communication
interface. When setting up the panel to communicate to a master device (likely a desktop
computer), the unit is considered a slave device for Modbus. It is important that both the
master device and the slave device are using the Modbus TCP/IP protocol (Figure 1). If the
master device does not use the Modbus protocol, then a protocol conversion device must
be used.
Figure 1. Conceptual block diagram of Modbus communication from master device to
slave device with data displayed to user interface.
1.1 MODBUS TCP/IP Frame Format
The Modbus TCP/IP (or simple Modbus-TCP) is the Modbus RTU protocol with a TCP
interface that runs on Ethernet. The Modbus-TCP message is simply a standard Modbus
RTU data embedded into a TCP message (Figure 2). The message content of TCP/IP is
equipped with checksum methods; hence, the Modbus checksum CRC is removed from
the original the Modbus application PDU used in serial Modbus. The Modbus TCP
transactions are functionally equivalent to serial counterparts with master and slaves
exchange PDUs. Consequently, a Modbus TCP PDU includes the Modbus Application
Protocol (MBAP) in addition to the traditional serial Modbus PDU. The MBAP header adds
four fields, transaction identifier, protocol identifier, length and unit identifier.
• The transaction identifier allows devices to pair transaction requests and replies.
• The protocol identifier indicates the application protocol encapsulated by the
MBAP header (zero for Modbus).
• The length field indicates the length in bytes of the remaining fields (unit identifier
and PDU).
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