Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 62)

4–8 Using Full-duplex Protocol to Send and Receive Messages
Publication 1770Ć6.5.16 - October 1996
The following program describes the actions of the receiver in detail.
RECEIVER is defined as
variables
LAST-HEADER is 4 bytes copied out of the last good message
RESPONSE is the value of the last ACK or NAK sent
BCC is an 8-bit block check accumulator
LAST-HEADER = invalid
LAST RESPONSE = NAK
loop
reset parity error flag
GET-SYMBOL
if DLE STX then
begin
BCC = 0
GET-SYMBOL
while it is a data symbol
begin
if buffer is not overflowed put
data in buffer
GET-SYMBOL
end
if the control symbol is not a DLE ETX then send DLE
NAK
else if error flag is set then send DLE NAK
else if BCC is not zero then send DLE NAK
else if message is too small then send DLE NAK
else if message is too large then send DLE NAK
else if header is same as last message send a DLE ACK
else if message sink is full send DLE NAK
else
begin
send message to message sink
send a DLE ACK
save a last header
end
end
else if DLE ENQ then send LAST-RESPONSE
else LAST-RESPONSE = NAK
end loop
GET-SYMBOL is defined as
loop
GET-CHAR
if char is not DLE
begin
add char to BCC
return the char and data flag
end
else
begin
GET-CHAR
if char is a DLE
begin
add char to BCC
return DLE and data flag
end
else if char is an ACK or NAK send it to the transmitter
else if char is an ETX
begin
GET-CHAR
add char to BCC
return ETX with a control flag
end
else return char with a control flag
end
end loop
The receiver keeps a record of the last response sent to
the transmitter. The value of this response is either DLE
ACK or DLE NAK. It is initialized to DLE NAK. When a
DLE ENQ (enquiry) is received from the transmitter, the
receiver sends the value of the last response.
The receiver ignores all input until a DLE STX or DLE ENQ
is received. If anything other than a DLE STX or DLE ENQ
is received on path one, the receiver sets the last response
variable to NAK.
When this symbol is received Then the
from the transmitter
DLE ENQ
last response is sent
and the receiver
continues waiting for
input.
DLE ACK BCC/CRC and the
message buffer are
reset, and the receiver
starts building a reply
message.
While building a reply message, all data symbols are
stored in the message buffer and added to the BCC/CRC.
If the buffer overflows, the receiver continues summing the
BCC/CRC, but the data is discarded.
If a parity, overrun, framing, or modem handshaking error is
detected, it is recorded .
If a control symbol other than DLE STX or DLE ETX is
received, the message is aborted and a DLE NAK is sent
to the transmitter. When DLE STX or DLE ETX is received,
the error flag, the BCC/CRC, the message size, and the
address (optionally) are all checked. If any of the tests fail,
a DLE NAK is sent.
GETĆCHAR is defined as an implementationĆdependant
function that returns one byte of data from the link
interface hardware