An I/O base module (14, 16, 18, 60, 100, 202, 204, 206, 208, 504, 506, 604, 606) for an industrial system comprising: an enclosure providing left (61, 102) and right (62, 104) flanking sides; releasable electrical connectors (64, 106, 108) supported on the left and right flanking sides exposed to connect adjacent base modules on the left and right sides; a releasable mechanical connector for holding the base module to adjacent base modules on the left and right sides; a plurality of electrical terminals (70, 120) for receiving conductors exposed on a surface of the enclosure to releasably retain electrical conductors communicating with an I/O function card (290, 292) held within the enclosure; backplane circuitry held within the enclosure and communicating with the releasable electrical connectors to establish a ring network communicating among attached base modules to provide a first port at one electrical connector and a second port at the second electrical connector so that the base modules may be arranged in a ring wherein the first and second ports of each base module couple to a different base module of the ring; and a bus slot for releasably receiving the I/O function card having a switching node (412, 432), wherein the first port and the second port are coupled to the switching node when the function card is installed in the bus slot, and the first port is coupled to the second port when the function card is absent from the bus slot, wherein the switching node is an active ring supervisor adapted to operate to: (a) transmit beacon frames out of each port at a first rate and monitor the other port for reception of beacon frames, wherein the beacon frames indicate a state of operation of the ring network as a closed mode or an open mode; (b) respond to a receipt of beacon frames between the first and second ports by blocking data frames between the first and second ports in an open mode; (c) respond to failure of the beacon frames to traverse the ring by changing to a closed mode passing data frames between the first and second ports. an enclosure providing left (61, 102) and right (62, 104) flanking sides; releasable electrical connectors (64, 106, 108) supported on the left and right flanking sides exposed to connect adjacent base modules on the left and right sides; a releasable mechanical connector for holding the base module to adjacent base modules on the left and right sides; a plurality of electrical terminals (70, 120) for receiving conductors exposed on a surface of the enclosure to releasably retain electrical conductors communicating with an I/O function card (290, 292) held within the enclosure; backplane circuitry held within the enclosure and communicating with the releasable electrical connectors to establish a ring network communicating among attached base modules to provide a first port at one electrical connector and a second port at the second electrical connector so that the base modules may be arranged in a ring wherein the first and second ports of each base module couple to a different base module of the ring; and a bus slot for releasably receiving the I/O function card having a switching node (412, 432), wherein the first port and the second port are coupled to the switching node when the function card is installed in the bus slot, and the first port is coupled to the second port when the function card is absent from the bus slot, (a) transmit beacon frames out of each port at a first rate and monitor the other port for reception of beacon frames, wherein the beacon frames indicate a state of operation of the ring network as a closed mode or an open mode; (b) respond to a receipt of beacon frames between the first and second ports by blocking data frames between the first and second ports in an open mode; (c) respond to failure of the beacon frames to traverse the ring by changing to a closed mode passing data frames between the first and second ports.