Ethernet I/O units may timeout when used with PAC Control or ioControl with control engine firmware R7.0a or newer, or with an OptoTerminal G75 or G70. It may also be seen when accessing an I/O unit or an I/O point using PAC Control or ioControl Debug mode.
- Control engine message queue shows intermittent I/O unit retry messages (-539) or "I/O Unit Timeout" messages.
- Control engine to I/O unit communication intermittently slows down due to retries.
The affected Ethernet I/O units can only receive up to 5 UDP messages at once. More than that, and messages are dropped and cause application retries or timeout errors. The 5 message limit affects any UDP port on the I/O unit, including port 2001 (MMP) and port 161 (SNMP).
The number of concurrent UDP messages sent by various applications and devices are listed. Use them to estimate the total number of UDP messages received simultaneously by your Ethernet I/O units.
Number of concurrent UDP messages sent by:
- PAC Control strategy One concurrent message per chart communicating to a brain
- PAC Control Debug mode (host task) One concurrent message when accessing an I/O unit or I/O point
- OptoTerminal-G75 or G70 One concurrent message
Example of a system exceeding UDP message limit to a single brain:
- A system with a PAC Control strategy having 6 or more charts (6 or more messages) communicating to a single brain.
- A system with a G75 (1 message), 4 charts (4 messages), and 1 PAC Control Debug session (1 message) communicating to the same I/O unit.
Opto 22 has resolved this issue by increasing the UDP buffer to 15 UDP messages.
Communication on the TCP port is not affected by the UDP message limit. For this reason, the problem is not affected by PAC Display, OptoOPCServer, or PAC Manager.