Attempting to multiplex a UART interface

Question

I'm working on a project that involves multiplexing the UART interface so that a single device can talk to many. My original plan was to use transistors to direct which child device the parent is talking to.

Any number of transistors(I'm using 2N3904 NPN) are connected at their collector pin to the parent device's TX pin, the transistor's emitter pin obviously going to the RX pin of the child. Then a matching set of transistors in reverse, collector to the child's TX pin & emitter to the parent's RX. A final set are connected at their collector to a regular digital pin on the parent with the emitter leading to the reset pin of the child.

Now another regular digital pin of the parent is used to control which of any group of 3 transistors are called(an RX, TX, and Reset connected transistor all connected to the same child). This configuration resembles SPI but saves on two wires.

Legend:

Yellow - Parent RX

Green - Parent TX

Orange - Reset Control Pin

Purple - Device 1 selector pin

Cyan - Device 2 selector pin

Prior to setting up the above diagram I tested my code and could reliably send/receive information with one child and no transistors. Then just inserted the transistors between the devices. That went from working perfectly without the transistors to only working some of the time. I've also confirmed that that reset pin works as intended through the transistor.

The problem is with the serial communication. Sometimes the commands the parent sends to the child work, sometimes they don't. and the information returned from the child is quite frequently distorted.

Is it possible to achieve what I am attempting here? Do I just need to use a different type of transistor or is there something inherent to how they work that transistors will always interfere with a communication signal?

Answer 1

There are a multitude of issues with your design/schematic:

• Never tie power from each unit together, as each regulator can be off and will cause unnecessary current loops. To power your level-shifter, just choose one Nano 5V supply connected to HV, and the 3V3 from the RaspPi connected to LV. Do not connect the Nano 5V to the RaspPi 5V.
• GND lines must all be connected together, which you have done.
• The top right transistor is missing a wire connected to the collector pin.
• The reset control to each Nano is tied to the RX function, but shouldn't.
• PNP transistors are a type of Bipolar Junction Transistors (BJT) and are not the same as MOSFETS, but your design uses them in this mode. This is likely the source of your bad communications, when the BJT is turned off then there is nothing to drive the signal high on the collector, so the UART will interpret noise as data.

If you want to use transistors for the switching function, then try using N channel MOSFET instead. Better yet, use a 74HC125 chip which contains four tri-state logic gates, enough for two Nano units.

Here is one solution:

Note that the Nano selection function pins (GPIO23 and GPIO24) must be inverted logic, so program them for '1' to unselect the Nano, and '0' to select.

The 74HC125 is okay with 3.3V or 5V inputs, so there is really only the RXI input to the RaspPi that needs shifting from 5V to 3.3V for its protection. Selecting an alternative 3.3V Arduino to the 5V Nano would completely eliminate the need for the level shifter.

Those pull-up resistors are needed when none of the Nano's are selected, to keep the signal high and noise free.