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Planing the Hardware Topology

In a complex scenario you will have various applications (unrelated GPIO sources) in your facility. For each application, a number of equipment provide GPOs while others receive GPIs from the former. It is also likely to have some equipment providing both GPI and GPOs but that is irrelevant to our purposes.

Equipment conforming a given application connect their GPIOs to units attached to a single BUS. In the unlikely case that more than 14 units would be required for a particular application, more buses can be assigned to it. What you can not do is to mix units from different applications in the same bus.

The first step in designing your system would be to define the hardware topology.

Units must be close to equipment they connect to, because this simplifies wiring. You must allocate units with suitable configurations near those equipment. For example, if your are implementing a Production Tally System, a unit with just GPOs would be placed near the camera CCUs, a similar one near tally-capable video monitors or multiviewers, and some units with just GPIs near the Production Video Switcher.

Units might be scattered among the facility but the GPIO wiring will be short and simple this way. From each unit, a serial cable connects to the serial BUS.

The Serial Bus is nothing but a panel with 15 DB9 connectors wired in parallel. One of those connects to the Master Linux hosts; the others, to PI units.

It is highly recommended to use a networked Device Server equipment instead of connecting the BUS to computer physical serial ports (via RS485/422-to-RS232 adapter, of course). There are several advantages on doing that:

* Facilitates future expansion of the topology.
* Removes the need for (not always reliable) RS485/422-to-RS232 adapters.
* Allows the Master Linux host to be virtual. This is specially interesting if you opt for an automatic-fail-over arrangement using two hosts.