TimeCritical Message Set
Chapter 9
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command specifies a word address. This field is in each command
message except a command to read an immediate-access block. The size
of this field is variable (2 through 11 bytes). The first byte specifies the
levels of addressing that are specified in the remaining bytes of the field.
Refer to Appendix B for an explanation of logical addressing.
SIZE — This one-byte field specifies an even number of bytes of data to
be read. This field is only in a read block command.
DATA — This field is limited to a maximum of 21 words for a
command to read an immediate-access block and limited to less than that
for other commands. Refer to each specific message format described
later in the chapter for the size and form of the data field for each specific
message type.
In the previous section we described the time-critical message fields
defined at the application layer. Here, we describe the format of each
specific type of time-critical command message and reply message at the
application layer. In each of these formats, we also show the time-critical
fields to indicate where the application-layer fields fit in the message
packet. Each time-critical write is a protected command. That is, if its
destination is as PLC-2 programmable controller, it can only access an
area of data table memory designated by a memory access rung.
Read ImmediateAccess Block
Use this command to read an immediate-access block in a remote node
interface for a programmable controller. The programmable controller
can establish a maximum of 16 immediate-access blocks (00 through 0F
hexadecimal) and update them periodically. The value in the USER field
specifies which of 16 possible blocks is to be read. When the
programmable controller’s interface receives this command, it
immediately sends the reply without waiting for the token. For all other
types of messages, the station executing the command must wait until it
receives the token before it can transmit a reply.
Application Message Formats