An industrial control system provides a programmable logic controller communicating with I/O modules which provide an electrical interface to controlled equipment and which support web servers for communication of I/O data directly with the Internet. The programmable logic controller communicates with the I/O modules over a control-network which may be separate from or shared with the Internet communications. A lock-out system prevents collisions between web clients writing to I/O and allows the programmable logic controller to limit web access to critical I/O points.