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4 |
Operation of the Communications Request
The figure and text below explains how a Communications Request is executed. The figure specifically illustrates the operation of an Establish Read Channel Command.
Domain of a TCP connection
Domain of a channel
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Series 90±70 |
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| Ethernet |
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PLC CPU | Backplane | Interface | LAN |
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Power flows to COMMREQ |
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in ladder program |
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Command Block sent to |
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| Verify |
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Interface |
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| Command Block |
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| and set up channel |
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| to server PLC |
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| Read Request |
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Data |
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| Data |
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| Return COMMREQ |
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| Status Word (CRSW) |
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COMMREQ |
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Status Word |
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| Pulse Data Transfer bit |
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Pulse received |
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| Read Request |
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Data |
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| Pulse Data Transfer bit |
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Pulse received |
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. |
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| Read Request |
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Data |
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| Data |
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| Pulse Data Transfer bit |
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Pulse received |
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Server
Series 90 PLC
Data
Data
Data
Figure 4-2. Operation of the Communications Request for an Establish Read Channel-
Command
1.A Communications Request begins when there is power flow to a COMMREQ func- tion in the client PLC. At this time, the Command Block data is sent from the PLC CPU to the Ethernet Interface.
2.For the Establish Read Channel Channel command, the COMMREQ Status word (CRS word) is returned immediately if the Command Block is invalid. If the syntax is correct, then the CRS word is returned after the next significant event: upon failure to establish a channel correctly and in a timely manner or upon the first successful trans- fer of data.
Once the channel is successfully set up to the server PLC, the Ethernet Interface performs the periodic reads as specified in the Command Block.
Chapter 4 Programming Communications Requests |