Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 40)

3–3 Using Half-duplex Protocols to Send and Receive Messages
Publication 1770Ć6.5.16 - October 1996
Each node on a multidrop link contains a software routine to transmit
and receive messages. DH and DH+ interface modules already
contain a slave transceiver routine, so they can be configured to
function as slave nodes in half-duplex mode.
Instead of a single routine, you can program separate transmitter and
receiver routines. However, in this chapter, we assume you are using
a single routine. The master and slave transmitter/receivers are
illustrated in the figure below:
source
sink
packet
OK
physical link to other slaves
master transceiver
slave transceiver
source
sink
packet
OK
packet
OK
packet
OK
(The “source” and “sink” are defined in the next section,
“Half-duplex Protocol Environment.”
To define the environment of the protocol, the transceiver:
•
needs to know where to get the message it sends, the message
source. We assume the message source:
– supplies one message at a time upon request from the
transceiver
– requires notification of the success or failure of the transfer
before supplying the next message
•
must have a means of disposing of messages it receives, the
message sink
When the transceiver has received a message successfully, it attempts
to give it to the message sink. If the message sink is full,
the transceiver will receive an indication that the sink is full.
Transmitter and Receiver
Message Transfer
HalfĆduplex Protocol
Environment