Emerson DL8000 - Page 132

DL8000 Preset Instruction Manual
D-4 Communications Protocols Revised February-2016
The maximum query message length for Modbus RTU is 256 bytes
(including check characters). The same length is true for response
messages.
In DanLoad 6000 protocol, a normal query and response frame consists
of the following fields:
 Address (one character)
 Function code (one character) – alternating hexadecimal 0x41 and
0x42.
 Data field length = N (one character) – including this character and
data field but excluding checksum bytes
 Data field (=N-1 characters where first character is a command code
and subsequent characters area data for the command)
 BCC checksum (two characters)
The exception response frame has the following format:
 Address (one character)
 Function code (one character) – 0xC1 and 0xC2 to indicate
“exception responses” to function codes 0x41 and 0x42 respectively
 Data field length = N (one character)
 Data field (=N-1 characters where first character is a command code
and subsequent characters are data for the command)
 BCC checksum (two characters)
For further information, see Section D.5, DanLoad 6000 Protocol
Frame.
The destination station uses block check characters (BCCs) to verify the
accuracy of the data received. The receiving station calculates the BCC
for the command query message it receives and then compares the
calculated BCC with the received BCC. Any differences indicate a data
error; the destination does not reply to such command frames.
The DL8000 knows which function code to expect from the TAS. If it
receives hex 41h when it expects hex 42h or vice versa (and the
command is not a Start Communications command), the preset assumes
a retry by the TAS and retransmits exactly the last response sent to the
TAS on the communication channel.
Additionally, the DL8000:
 Uses function codes C1h and C2h to indicate “exception responses”
to function codes 41h and 42h, respectively.
 Does not respond to a “broadcast” messages (that is, where the
address field equals zero).
 Operates as an addressable slave station, and “speaks only when
spoken to.”
The DL8000 does not respond to a message that is “incorrectly framed”
(that is, not its address, an unused function code, or an incorrect error
check). It also does not respond to a message that is “incomplete”
(containing either fewer or more characters than expected).