In SX Microcontrollers, SX/B Compiler and SX-Key Tool, robotworkshop wrote:

It is best to keep the runs as short as possible when using the TTL (Logic level) serial signals.  If you want long distances then go ahead and use a MAX232 (or similar device) for Serial or use one of the FTDI adapters already listed.

As far as sharing the line there are certainly ways to do it is you are careful.  The RX signal can go to multiple devices easily.  A few things to consider is the fan out so that you are not trying to drive more devices than the driving device can handle.  If you need more you may need a driver to buffer the signal so it can drive more things.  In this case it may be more like a Party line so all the devices hear the signal.  This is fine if you are using something like the AppMod style protocol so it will only respond to data sent to the particular device.  You may also want to use some selection logic so that only one device at a time hears what is coming down the Serial line.  If you get a chance to read the April 2008 issue of SERVO I did something like this for my BOB robot project.

For the multiple devices sharing the TX line you can either use some extra logic as a MUX so that only one can use the line at a time.  Another option is to use an "open-collector" as the TTL TX on all your devices and have a pull-up resistor on the line.  This way any of the devices can talk without shorting anything out.  Of course they should be well-bahaved and not all try to talk at once.  Again, the AppMod protocol is useful here since the target device is typically asked for information and only that particular device would respond.

If you are using a USB adapter I would certainly look at sharing the logic level serial line with several devices and look at all the examples that use the AppMod protocol.  Parallax already has many well documented and several are done with SX/B on the SX chips.  Projects like that are one example where the SX processor is an excellent choice.

Hope this helps,

Robert

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