Important: Retries and Negative acks from a C-Bus PCI or SIM

This information will soon make it into an update to the C-Bus Interface Requirements document, but we want to make this known to members of the C-Bus Enabled programme.

Retries and the C-Bus PCI / SIM

If a transmission onto C-Bus by the PCI or SIM fails, the PCI / SIM automatically retries that transmission.

It is not necessary for customer software to apply their own additional retry methods over the top of the C-Bus PCI / SIM.


Transmission to a non-existent address

The PCI / SIM can return a transmission status indicating the transmission was dropped. (It will return a # character). A dropped transmission indicates that the destination application address does not exist on the network.

A device that continually transmits to a non-existent destination has the potential to waste C-Bus bandwidth, and the potential to delay other useful transmissions (due to the dropped transmission being retried a number of times before being abandoned).

Clipsal strongly recommend that the status of transmission attempts to C-Bus be checked, and the following algorithm be applied:

• count the number of successive transmissions (on a per-application address basis) which are dropped
  
• when there are 3 successive transmissions to the same application address which are dropped, cease all further transmissions to that application address (either until the power is recycled or ? see below)
  
• the counter of successive dropped transmissions is cleared every time there is a successful transmission to that application address, and on power up
  
• the counter of successive dropped transmissions (per application) is cleared if any message is received from that application address.
  
• OPTIONALLY, once per hour, all of the counters may also be cleared