Sequencing Communications Requests; Managing Channels And Tcp Connections - GE RX3i User Manual

Chapter 7. Programming SRTP Channel Commands
Rung # 1: Input LANIFOK (bit 16 of the LAN Interface Status bits) monitors the health of the Ethernet Interface.
If it is OK to send a COMMREQ, the LAN_OK coil is ON. LAN_OK is used as an interlock for Rungs 3–6.
Rung # 2: Input BEGREAD triggers READREQ, which enables execution of the MOVE and COMMREQ functions.
READREQ is a one-shot (Positive Transition) coil, activating once when BEGREAD transitions from OFF to ON.
Rung # 3: The MOVE WORD function moves a zero to the COMMREQ status word referenced in the Command
Block (see rung #4). This clears the COMMREQ status word. This rung also resets the FT output coil of the
COMMREQ Function Block in rung #6.
It is vital that the COMMREQ status word be cleared and the COMMREQ fault output coil be cleared each time
before initiating a COMMREQ function.
Rungs # 4–5: The BLKMV INT functions set up the COMMREQ Command Block contents. When these rungs are
activated, the constant operands are moved into the memory beginning at the address indicated in the
instruction. The constant operands in this example are defined in the Establish Read Channel Example in this
chapter.
Rung # 6: The COMMREQ Function Block.
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The IN field points to the starting location of the Command Block parameters (%R00301 in this example).
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The SYSID field of the COMMREQ function block defines the rack and slot of the Ethernet Interface to
receive the command data. This is a hexadecimal word value that gives the rack (high byte) and slot (low
byte) location of the Ethernet Interface module. In the example ladder diagram shown, the first three
number places (from left to right) are zeroes and are not displayed; only the last number, 4, appears. This
indicates rack 0, slot 4.
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The TASK field of the COMMREQ function block indicates which mailbox task ID to use for the specified rack
and slot. For a PACSystems rack-based Ethernet module, Task must be set to 0. For a PACSystems CPU
embedded Ethernet interface, Task must be set to 65536 (10000H).
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The FT output (energizes the FAULT coil in this example) is turned ON (set to 1) if there were problems
preventing the delivery of the Command Block to the Ethernet Interface. In this case, the other status
indicators are not updated for this COMMREQ

Sequencing Communications Requests

If the Ethernet Interface receives Command Blocks from the PLC CPU faster than the Interface can process
them, the Interface will log an exception event 08, Entry 2=0024H and will log the PLC Fault Table entry:
"Backplane Communications with PLC Fault; Lost Request"
Only one COMMREQ function per channel can be pending at one time. A COMMREQ function is pending from
the time it is initiated in the ladder program until its COMMREQ status word has been updated to a non-zero
value by the Ethernet Interface.
If the PLC CPU attempts to send COMMREQs to the Ethernet interface faster than the Ethernet interface can
receive them, the FT output of the COMMREQ function block will be set and the CPU will generate the following
entry in the PLC Fault Table:
"Mailbox queue full – Comm_req aborted"
The PLC logic program should send retry the COMMREQ after a short delay.

Managing Channels and TCP Connections

When you issue a COMMREQ to establish a read or write channel, a TCP connection is created, the transfer(s)
are made, then upon completion of all the transfers, the TCP connection is terminated. It takes time to create
and to terminate these connections. If an application is constructed so that it rapidly and repeatedly
establishes a channel with only one repetition (one transfer), the available TCP connections for the Ethernet
Interface may be totally consumed. A "snapshot" of the state of the TCP connections would show some of
them being created, some being terminated, and some active, but none available.
130 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224Q