Synchronization of actions between different nodes in a mesh?

[Mitch996]

Hi, when reading BLE Mesh specs, this paragraph caught my attention:

Messages may support a delay parameter that indicates a delay between receiving a message and starting the state transition. This helps when synchronizing actions of multiple receivers (such as lights) when senders retransmit messages multiple times. Each retransmitted message may indicate a different delay, compensating for the time elapsed since transmitting the first message.

My project could totally use this feature, I'm wondering how this can be done precisely (<=20 ms if possible), that is, let's say I need 6 motors to start rotating at the same time, or else major mechanical mishaps take place, how do I do that, by embedding a hop count of some sort in the message sent over the mesh? Or is there any other way which I am not aware of?

And above everything, how reliable is this? Since if one node fails to receive the message THE FIRST TIME, the outcome could be very, very undesirable?

[David Edwin]

You want a message with ack if you want reliability, you also want to ensure that all motors have got the message before starting up. You may also want a fallback to cancel and restart if one of the nodes did not ack within your timeout. The mesh already has a hop count that can be used for re-transmits.

You can use the mesh for this but since the node count is low (only 6 nodes), it is also possible use a simple scanner/advertiser to achieve this level of synchronization.

All motors can be started within 20ms of each other with sync either on the 16MHz or 32KHz clocks

[Mitch996]

You are right, I'm not there yet, but so far I'm thinking about utilizing the "shockburst" TM feature: find out the "central" (spacially speaking) node, and assign her with the task of sending out synchronizing bursts periodically. The Shockburst has a max range of 200m, which is more than enough for us.