Plan your process networks long before you need them.
We have all seen the situation: a new piece of equipment has to be connected to some network right now, because the facility manager has labeled it critical and it must appear on the monthly reports. So it gets tied into the nearest network, or perhaps an odd gateway is installed to connect it to an existing system, or maybe we have to install an extra communications module to let it talk over some other protocol. Next month, we go through a similar process when a new interlock between unconnected equipment needs to be installed, so we rig something up to get from A to B. Follow this process for a few years and the process control network diagrams begin to look like spaghetti.