Rockwell-automation 1770-KF2 Data Highway or Highway Plus Interface Mo Manual de usuario descargar pdf (Pagina 58)

Asynchronous Link Protocols

Chapter 4

4-15

able 4.B

Receiver

for Full-Duplex Protocol

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-CODE

if DLE STX then

begin

BCC = 0

GET-CODE

while it is a data code

begin

if buf

fer is not overflowed put data in buf

fer

GET-CODE

end

if the control code 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 last header

end

else if DLE ENQ then send LAST

-RESPONSE

else LAST

-RESPONSE = NAK

end loop

GET

-CODE is defined as

loop

GET-CHAR

if char is not a 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 a DLE and a 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 character with a control flag

end

end loop

GET

-CHAR is defined as

an implementation-dependent function that returns one byte of data from the link interface hardware.

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