ModBus Plus Dual Cable Phantom Errors
The advantages of utilizing the dual cable capability of the Modicon Modbus Plus system are to increase and maintain system uptime and availability. The dual-redundant nature of the system is virtually invisible to the user. Messages are transmitted on both a primary cable, (A) and a secondary cable, (B) simultaneously. The messages are received on both cables as well.
All message statistics are logged and attached to the message token. If any errors occur, the red error LED will illuminate at both the receiving and transmitting nodes/devices and the error log will indicate the type of error and upon which cable the error occurred.
A dual cable system can also have single cable nodes/devices connected to it. These nodes/devices and the network will function perfectly with the mixture of dual and single cable devices. Of course, the error LED on the transmitting node/device will illuminate if there is no cable B attached to a single cable device.
As long as the message is received on one of the cables, the users’ application will function as it normally would, keeping the application online.
This system greatly reduces the chances of a complete application failure due to a cable issue.
However, when monitoring, checking and testing these dual cable systems that also have single cable devices installed, using MBP Stat, it is very important to utilize a DUAL network interface card, such as an SA85-002 card. If single cable cards, such as an SA85-001 or a PCMCIA card are used, the error statistics reported by the single cable card to the MBP Stat program would be incorrect! The single cable card has no error routines for the dual cable, so it incorrectly reports any errors when the dual cable data is mixed with its’ single cable error routine.
If you must utilize a single cable network card when monitoring or testing a network, you can only check ONE cable at a time. The statistical information for a single cable will be correct.