No-library single-byte serial transmit?

I am receiving serial stream from a device at 2400 baud (device does not even have a RX line). The contents rarely changes, but when it does, once in every 5-30 s, I need to send (i.e. only TX) a single byte to one of the two bluetooth devices, depending on the changed byte in the device stream. I am using either Arduino Nano or Pro Mini (I have both, and I tried both), attaching the device to Serial, and bluetooths to digital pins over two Software Serial objects at 38400 bps.

The device stream reading works fine alone, and sending fictitious data to the two bluetooths works fine on it's own, but when all is in one code, I start missing bytes. I suspect the Software Serial interference. I have tried using bluetooths at 9600 bps, I also tried using Software Serial objects for all 3 connections, and I still loose bytes.

Now I need help. What came to my mind is a custom function that only sends a single byte over a digital line to the RX line of one of the two bluetooth modules, so I can drop the Software Serial library that I believe is causing the byte drop on the incoming stream. Anyone has, or have seen such a code?

The SoftwareSerial class blocks all interrupts on the Arduino while receiving or sending a byte. The class is only able to receive data on one instance at a time, so you have to know when data arrives on the different instances or you loose data. With SoftwareSerial you will always have this kind of problems, you may be able to lower them a bit but in the end you'll never get a reliable system. I'd suggest to use a Teensy 3.x which offers you 3 independent hardware UARTs (the ones from Teensy 3.1 should be even 5V tolerant) and you can program them also with the Arduino IDE. Also they are about the same size as the Arduinos you're using.

That is now what I was asking for, but thank you for the comment and the idea!

In the meantime, I managed to use SoftwareSerial for all 3 connections, after realizing that the order in which you initialize SoftwareSerial objects matters (peeking into the TwoPortReceive example helped).