Cutler-Hammer D320 PLC, User Manual

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D320 PLC User's Manual
Communication System Registers
In addition to the special Flags used to configure communications, a bank of System Registers (SR298-
SR373) is provided for holding the data transmitted and received. The descriptions of the system
registers are contained in Table 3 below.
Table 3 – Communications Registers
System Registers Description Explanation
SR298-SR333 Transmit Data Buffer Holds up to 36 words of data to be transmitted
SR334-SR369 Receive Data Buffer Holds up to 36 words of received data
SR370 Transmit Data Length Number of Bytes to be transmitted
SR371 Receive Data Length Number of Bytes that have been received
SR372 Start Code Start code for ASCII Comm.– one byte, high byte unused
SR373 End Code End code for ASCII Comm. – one byte, high byte unused
Descriptions of Configuration Flags and Registers
Each of the Flags and Registers has a specific purpose, based on the mode of communications. The
following paragraphs describe in greater detail the operation and use of each flag and register.
F11.0 REQUEST TRANSMISSION
UDCP Mode. Once the ladder program has filled the Transmit Data Buffer (SR298 – SR333), and set
the number of Bytes to send (SR370), the program sets this flag to indicate to the CPU that it is time to
send the data. Once the CPU has sent the number of bytes indicated, this bit is automatically reset by
the CPU.
F11.1 TRANSMISSION FAILURE
UDCP Mode. If the CPU encounters a failure in transmitting the data indicated (e.g. the number of
Bytes to send > 72), this flag is set. This flag is not automatically reset, and must be reset by the user
program after each occurrence.
F11.2 ENABLE START CODE
UDCP Mode. In the ASCII mode of communications, it is possible to define a “start code” to signal
the beginning of a message. Setting this flag enables the Start Code mode of operation. When set, the
CPU will look for the Start Code (SR372) on any message received before storing the data into the
Receive Data Buffer (SR334-SR369).
F11.3 ENABLE END CODE
UDCP Mode. In the ASCII mode of communications, it is possible to define an “end code” to signal
the end of a message. Setting this flag enables the End Code mode of operation. When set, the CPU
will look for the End Code (SR373) on any message being received. When the End Code is received,
the CPU stops storing the incoming message, sets the Receive Data Length (SR371), and sets the
Message Received Flag (F11.4).