Resilient data communications system

A communication data system is designed for resiliency by automatically replacing or bypassing defective units. The system includes a number of input/output terminals which are connected to MODEMs through a relay bank. The MODEMs send serial data to a serial I/O module which converts the serial data to bytes which it places on a VMEbus. A network processor sends the data from the VMEbus to a general purpose computer which places the data into the communications network. A general purpose computer or a back-up general purpose computer may detect a defective communication link and automatically switch to a back-up network computer, or cause a control module to switch the relay module to a spare MODEM and spare SIO. The control module may also generate a remote line test to the link between the MODEM and the terminal to determine if that link is defective.