Chapter 5. Ethernet Global Data92 PACSystems* RX7i & RX3i TCP/IP Ethernet Communications User Manual GFK-2224QModifying Exchange Variables on Targets that use EGD CommandsPACSystems targets can service EGD commands from other devices. Some commands read or write anexchange based solely on an offset into that exchange. If EGD signatures are not used, the exchange offsetand length requested are validated against the length of the exchange. Without EGD signatures, the definitionof the exchange can be changed entirely by an RMS and the EGD command would be serviced as long as theoffset and length in the command are valid. For this reason, caution should be used when modifying EGDexchanges on a target that services EGD commands. Adding variables to the end of such exchanges would notcause a problem, but modifying or deleting variables should only be done with caution.PACSystems targets can also be EGD command clients. EGD commands can be sent to other devices viaCOMMREQs in user logic. If EGD will be modified using RMS, the exchange signature should be set to thesignature value of the device that will service the command. Do not set the signature value to zero, thiseffectively disables signature checking.5.7 Monitoring Ethernet Global Data Exchange StatusThe Exchange Status word is used to store status information about an EGD exchange. A unique ExchangeStatus word location must be is configured for each exchange.The PLC writes status codes into the Exchange Status word whenever an exchange is transferred or aconsumer timeout occursThe Exchange Status word is typically set to 1, indicating that data transfer occurred successfully. Theapplication program can monitor for error conditions reported in the Exchange Status word by setting it to 0once a non-zero value is written to it. In all cases, if the least significant bit of the exchange status is set to a 1,then data was transferred successfully. Status values other than 1 with the least significant bit set (e.g. 3, 5 and7) give information about the data that was transferred, the producer or the network that are noteworthy in theapplication.The program should also monitor the “LAN Interface OK” Status bit (see Chapter 12, Diagnostics) for each of theEthernet Interfaces performing EGD. The Exchange Status word is invalid if the bit is 0.Note that when an EGD exchange message received from the network contains an invalid Protocol VersionNumber, the Ethernet Interface cannot decode the message in order to identify the exchange. In this case, theExchange Status Word cannot be updated.
