Rockwell-automation 1775-KA PLC-3 Communication Adapter Module User Ma Uživatelský manuál stáhnout pdf (Strana 144)

Half-Duplex Protocol

Chapter 11

1117

If each station is polled only once per cycle, the master must keep a

record of the first 6 link–level data bytes of the last transmission from

each station, since other stations may transfer messages between

retransmissions from a given station.

Figure 11.9

Duplicate

Message T

ransmission

xxxx

Not Full

SINK MASTER LINK SLAVE SINK

10059–I

DLE ACK

SOURCE/ SOURCE/

BCCDLE ENQ STN

DLE STX xxxx DLE EXT BCC

BCCDLE ENQ STN

DLE STX xxxx DLE EXT BCC

Not Full

DL ??? CK

Sometime Later ...

(Discard Retransmission)

When a slave station fails to respond to a message from the master, you

should poll the slave to see if it is there. If it answers the poll with a DLE

EOT but consistently fails to ACK the master’s message, the slave’s

message sink is probably full. If the slave answers with DLE EOT to a

poll, you should wait for the slave’s receiver buffers to clear. This

situation is illustrated in figure 11.10.

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Rockwell-automation 1775-KA PLC-3 Communication Adapter Module User Ma Uživatelský manuál Strana 144