Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 42)

3–5 Using Half-duplex Protocols to Send and Receive Messages
Publication 1770Ć6.5.16 - October 1996
GETMESSAGE is defined as
GET-CODE
while it is data code
begin
if buffer is not overflowed put data in buffer
GET-CODE
end
if it is a control code and it is an ETX then
begin
if parity error flag is set then return a NAK
if BCC is not zero then return a NAK
if message is too small then return a NAK
if message is too large then return a NAK
return an ACK
end
else end
GET-CODE is defined as
loop
variable
GET-CHAR
if char is not a DLE
begin
add char to BCC
return the char and a data flag
end
else
begin
GET-CHAR
if char is a DLE
begin
add char to BCC
return a DLE and a data flag
end
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
end
GETĆCHAR is defined as
an implementationĆdependent function that returns one byte of data
from the link interface hardware
SEND (Message) is defined as
begin
BCC = 0
send DLE STX
for every byte in the message do
begin
subtract the byte from the BCC
send the corresponding data code
end
send DLE ETX BCC
end