Noninvasive testing of network interface error codes for safety network

A safety communications system comprising:

• a network for transmitting messages according to a network protocol each message has at least one of a header and a footer, the message includes a first error detection code, and a data segment, the data segment includes data and a second error detection code, the first error detection code derived from the data segment according to the network protocol;
• a network interface connectable to the network to receive the messages, the network interface including a network error testing means reading the first error detection codes of the messages to detect errors in the data segment of messages;
• and
• a supplemental error testing means communicating with the network interface to receive the data segment of received messages, the supplemental error testing means using the second error detection code to independently detect errors in at least a portion of the data of received messages;
• the supplemental error testing means monitors the message data segment passed by the network interface to assess the correct operation of the network error testing means without the use of dedicated test signals but by using a background error naturally occurring in the messages on the network to provide an indication of an error in the network interface;
• the supplemental error testing means is a comparator which compares the data of data segment against the second error detection code embedded in the data segment to detect errors not identified by the network error testing means;
• the second error detection code is at least one of a checksum of the data, a complementary copy of the data, and a checksum of the complementary copy of the data and the supplemental error testing means compares the data against the second error detection code.