Correct. But since we know that Mahmood used RS-422 and ethernet cables, we know that the state of Internet in Jordan would be irrelevant. In this case, the clocks are quite close together - within the distance you can span with RS-422. So there are no big and unknown ping time between master and slave to take care of. The delay is the baudrate delay and the delay from the code filling the transmitter and the code picking up data from the receiver.

(He's)Mahmood used this for RS-422

I'm transmitting from one source (talker) to other slaves (Listeners) in one direction only so I only need 2 wires.
Mahmood

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Edit: Or as an alternative have all slaves just contain a GPS as time reference - but that would add some extra troubles with daylight savings.

It will be better used GSM BTS clock reference.(I saw someone did that and sent the GSM syncronize clock to slave,but unfortunedly the Master MCU is 32-Bit and had USB host).

Thank you
Jeckson S Ben