Command/Reply - Allen-Bradley 1779-KFMR User Manual

Command/Reply

Communication is always initiated by a node transmitting a command
message. Only the node holding the token can initiate communication by
transmitting a command message. When a node transmits a command to
read an immediate-access block, the node receiving the command
immediately transmits a reply message while the initiating node still holds
the token. For any other command requiring a reply, the node receiving
the command must wait until it holds the token before it can transmit the
reply message.
Some commands call for the node receiving the command message to
transmit no reply message.
Reading a Block of Data
You can generate two types of commands to read a block of data:
Read Block
Read Immediate-Access Block
When a node interface receives a read block command from another node,
it reads the specified area of the host's memory; then, when it receives the
token, it transmits the reply message containing the block of data
specified in the command.
For a node to reply to a command to read an immediate-access block, the
host must periodically write a specified block of data from the host's
memory into a buffer in the node interface. When the node interface
receives a command from another node to read an immediate-access
block, it reads the specified block of data from its buffer and immediately
transmits the reply message without waiting to receive the token.
Writing a Block of Data
For write block commands, you specify a block of data from the local
host's memory that you want to write into a specified block of memory in
the host at a remote node.
For a write block command, you can specify either:
Write with No Reply
Write with Reply
Write Verified
Chapter 2
Introducing the 1779 KFM Interface
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