Hi Marco,
if we move to a TX driver that is not coupled in AC on the bus, the only way that we can follow is drive some current to the ground while transmitting. Because we can no longer impress a small voltage signal (5V) over a bigger (24V) bus voltage.
I agree that can be a solution to our problems, because impressing a voltage require a frequency in the MHz range, and this may lead to problems if the circuit is build over discrete components and not ICs.
There is one concern about this. If we drive some current to the ground, the only effect that we may expect is a lower voltage on the bus, that will depend on the internal resistance of the power supply.
Assuming a not regulated 30 VDC 1A power supply, we can assume an internal resistance of 5 Ohms, that will lead a 5V voltage decrease when supplying 1A. So we have to use the TX current to lower the voltage.
We have to identify a voltage drop that identify our bit over the bus, let me say 5V. That means, when the power supply is fully loaded (1A), the steady state voltage is 25V and while transmitting the voltage shall drop to 20V. Most of the 24V relays will work at 20V.
So, if the generator resistance is 5 Ohms, we need to drive 1 more ampere to the ground, I think that is too much. On the other hand, the transmission will be for shorter time, and the max current from the power supply is 6A (short circuit current).
Let me say that it may work, but doesn't look to me the best suitable option.
One way could be find out a voltage regulator circuit, that will increase the output resistance at high frequencies. In that case, the problem to address will be: 1) How to get this; 2) Does this affect the normal operation in case of load changes and noise?
A starting point could be this one.
What do you think about this?
Regards,
Dario.