Rockwell-automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command Instrukcja Użytkownika Pobierz pdf (Strona 58)

4–4 Using Full-duplex Protocol to Send and Receive Messages

Publication

17706.5.16 - October 1996

Figure 4.3 shows the protocol environment for message symbols

from transmitter A to receiver B (path 1) and response codes from

receiver B to transmitter A (path 2).

Figure 4.3

Protocol Environment

Transmitter A SinkReceiver BSource

Packet

Sink FullPacket Status

Path 1

Path 2

Packet

Ideally, the data-link-layer protocol is not concerned with the content

or form of the message packet (link-layer data) it is transferring.

However, full-duplex protocol places the following restrictions on

link-layer data submitted to it for transfer:

• minimum size of valid link-layer data is 6 bytes

• maximum size of valid link-layer data depends on the

application-layer command

• some protocol implementations (e.g., point-to-point links to a

1771-KG module) require that the first byte of the link-layer data

match the node address

The receiver ignores messages that do not contain the correct

address.

• as part of the duplicate message detection algorithm, the receiver

compares the second, third, fifth, and sixth bytes of the link-layer

data with the same bytes in the previous message

If there is no difference between the sets of bytes, the message is

classified as a re-transmission of the previous message. You can

set some Allen-Bradley interface modules so that they do not

implement duplicate-message detection.

In full-duplex protocol, messages are sent from the source (part of

software that supplies message packet) through a transmitter

(device that sends data) and then a receiver (device that receives

data) to the sink (part of the software that accepts the received data):

For See page

structured text on how the transmitter operates 4-5

a flowchart of how the transmitter operates 4-6

structured text on how the receiver operates 4-7

a flowchart of how the receiver operates

4-9

Message Characteristics

Transmitter and Receiver

Message

Transfer

1 2 ... 53 54 55 56 57 58 59 60 61 62 63 ... 250 251

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