The UART pins on the microcontroller are NOT tri-stateable.

For 100 devices all over on other boards you will have to provide buffers at each device for the UART lines. So if you have a problem with no more space on you small board then this is a lack of planning ahead.

If you want to wire the chain like you showed in your post then use RS422. This will afford a net work with a 4-wire serial bus (plus GND of course) where all the devices connect differentially.

There will be a need at each slave device for an additional I/O pin from the microcontroller to each RS422 driver that will be used to port that slave onto the lines back to the master.

It takes a proper packeting protocol to ensure orderly communication from the master to each slave as needed. Without this there would be massive contention on the RS422 bus as multiple devices tried to talk at once. The best protocols end up essentially using the bus in a half deplex manner, IE Master sends and then addressed slave responds.

You would owe it to yourself to seriously consider the use of a 2 wire bus like RS485. It is simpler to wire up and the already half duplex nature of a master slave protocol fits right on an RS485 link.

Note that RS422 and RS485 networks are typically limited by specifications to about 30 devices on the bus. In order to support your 100 devices you are going to have to setup 3 or 4 sub-busses and install repeaters at the appropriate points.

Now go make your board big enough to handle an additional SO-8 package and get some LT1785 samples from Linear Tech.

Michael Karas