Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 236)

14–4 Line Monitor Examples
Publication 1770Ć6.5.16 - October 1996
Command
Field
Value Function
DLE STX (2 bytes) 10 02 Indicates the start of a message
DST (destination) 09 Indicates the remote node address that the computer is communicating to. 09 hex equals 011 octal,
the address of the 1771ĆKA2.
SRC (source) 00 Indicates the Data Highway node address that is the source of the message. In this example, the computer
uses an SRC byte of 00 (hex). The asynchronous interface (1770ĆKF2) automatically inserts the correct
SRC value before sending the message across Data Highway to the 1771ĆKA2.
CMD (command) 01 Indicates the specific type of command. In this example, the CMD byte has the value 01 (hex) which
indicates an unprotected read of the PLCĆ2/30 data table.
STS 00 Indicates the status of the message. A command message should always set this field to 00 (hex).
TNS (transaction)
(2 bytes)
01 00 Indicates a specific transaction value for each message. The TNS value increments for each message
packet. This value makes each message uniquely different, which helps you check for duplicate message
packets.
ADDR (address)
(2 bytes)
11 00 Specifies the PLCĆ2 data table address where the unprotected read starts. The combination of these two
bytes points to a byte address. Since the PLCĆ2 works in word (2 byte) increments, the address should
specify an even number of bytes. The address must be transferred low byte first. In our example,
the address 11 00 (hex, low byte first) equals 000 022 octal byte address or 000 011 octal word address.
SIZE 02 Specifies how many bytes of PLCĆ2 data table information you read in this transaction.
DLE ETX (2 bytes) 10 03 Indicates the termination of the message.
BCC (block check
character)
E2 Used to check the accuracy of the message transmission. You can optionally use CRC bytes here.
The BCC value must equal the 2's compliment of the 8Ćbit sum of all data bytes between DLE STX and DLE
ETX. (For more information on BCC and CRC error checking, see page 5-4 .)
DLE ACK 10 06 Sent from the module (1770ĆKF2) back to the computer to indicate that the module successfully received the
message. If the BCC value calculated by the module does not match the BCC value in the message packet,
the module sends a DLE NAK (10 15) instead of the DLE ACK (hex).