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Slave Packet Formats
To invoke DH-485 link layer services, a special PCCC CMD byte is used
to support the addition of link-specific information to the packet. Setting
the CMD byte = 09 indicates that the packet contains extended link
information.
When the CMD byte = 09, the packet contains a "Link Type" field (one
byte), a "Link Status" field (one byte), and information specific to the
link-type command being sent.
Figure 4.3 shows the SRD request packet format.
Figure 4.3
The SRD Request Packet Format
DLE
STX
DST
SRC
Link Type (LTYP)
This one-byte field defines the Link Type as follows:
1
2-255
Link STS (LSTS)
This one-byte field indicates the Link Status of the SRD request. It is used
on replies to SRD requests only. When sending an SRD command, this
field should be set to zero.
00
01
02
CMD
LINK
LSTS
LINK PACKET
09
TYPE
INFORMATION
– SRD Link Request
– Reserved for future expansion
Success. The message was successfully sent to the destination
device. Note that some slaves will return data with the success
code (ACK with Data).
NAK, No Memory at Destination (SRD reply). This link error
is returned when the KF3 receives a NAK, No Memory message
from the destination station in response to an SRD request.
NAK, Undeliverable Message (SRD reply). This link error
is returned if the 1770-KF3 was unable to send the SRD to the
destination station (as may occur when the destination does not
exist on the network).
Chapter 4
Communicating with the KF3
DATA...
DLE
ETX
BCC/CRC
4 7
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