Opto 22

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KB52254
Ethernet I/O units drop UDP messages and may timeout if too many messages are received at once
Revision:  4.0
Published:  6/22/2010
Applies To
Firmware for:
SNAP-B3000-ENET
SNAP-ENET-D64
SNAP-ENET-S64
Versions
Problem affects all firmware versions including and prior to:  
R7.0a
Problem is fixed in version:  
R7.1i

PROBLEM

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.

Symptoms include:
  • 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.

CAUSE

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.

RESOLUTION

Opto 22 has resolved this issue by increasing the UDP buffer to 15 UDP messages.

MORE INFORMATION

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.

Relevant Downloads
No relevant downloads have been specified.
Questions? Contact Opto 22 Product Support.
Phone: 800-835-6786 or 951-695-3080
Email: support@opto22.com

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This Opto 22 Knowledge Base ('OptoKB') article is intended to provide general technical information on a particular subject or subjects and is not an exhaustive treatment of such subjects. Accordingly, the information in this OptoKB article is not intended to constitute application, design, software, or other professional engineering advice or services. Opto 22 may modify the OptoKB articles at any time. Before making any decision or taking any action which might affect your equipment, you should consult a qualified professional.

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