Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 59)

4–5 Using Full-duplex Protocol to Send and Receive Messages
Publication 1770Ć6.5.16 - October 1996
How the Transmitter Operates
The following program describes the actions of the transmitter:
TRANSMITTER is defined as
loop
Message=GET-MESSAGE-TO-SEND
Status=TRANSFER(Message)
SIGNAL-RESULTS(Status)
end loop
loop
WAIT for response on path 2 or timeout.
if received DLE ACK then return SUCCESS
else if received DLE NAK then
if nak-limit is exceeded then return FAILURE
else
begin
count NAK re-tries;
SEND-MESSAGE(message);
start timeout
end
else if timeout
if enq-limit is exceeded then return FAILURE
else
begin
count ENQ re-tries;
send DLE ENQ on path 1;
start timeout
end
end loop
SEND (message) is defined as
begin
BCC = 0
send DLE STX on path 1
for every byte in the message do
begin
add the byte to the BCC;
send the corresponding data symbol
on
path 1
end
send DLE ETX BCC on path 1
end
GET-MESSAGE-TO-SEND
This is an operating-system-dependent interface
routine that waits and allows the rest of the
system to run until the message source has supplied
a message to be sent.
SIGNAL-RESULTS
This is an implementation-dependent routine that
tells the message source of the results of the
attempted message transfer.
WAIT
This is an operating-system-dependent routine
that waits for any of several events to occur
while allowing other parts of the system to run.
TRANSFER (Message) is defined as
initialize nak-limit and enq-limit
SEND(Message)
start timeout
Whenever the message source can supply a message packet and the
transmitter is not busy, it sends a frame on the link to the destination
address. It then starts a timeout, and waits for a response.
When this response is
The message packet
received from the receiving
address
DLE ACK
has been successfully transferred.
After signaling the message
source that the message packet
was successfully transmitted, the
transmitter proceeds with the next
message packet.
DLE NAK is retransmitted. The transmitter
restarts the timeout and waits
again for a response. If it receives
a DLE ACK, the transmitter starts
a timeout.
This can be repeated several
times. You can set a limit to the
number of times a message can
be reĆtransmitted for each module.
If this limit is exceeded, the
message source is informed of the
failure and the transmitter
proceeds with the next message.
If the timeout expires before a response is received, the transmitter sends
a DLE ENQ to request a retransmission of the last response. It restarts
the timeout and waits for a response.
You can also set a limit to the number of timeouts that are allowed per
message. If the enquiry (ENQ) limit is exceeded, the transmitter signals
the message source that the transmission has failed, and the transmitter
proceeds to the next message.
There are three responses defined: DLE ACK, DLE NAK, DLE ENQ,
If the transmitter receives an different response, the transmitter ignores it.