After giving this a bit more thought I think it should be possible to modify the mqtt_schedule plugin to work with a specified range of stations.
You would just need to schedule all the stations for both SIP installations on the "master" SIP which would use the mqtt_zones plugin to broadcast all the zone states each time a station is switched on or off.
The "slave" SIP installation would be running the modified mqtt_schedule plugin (I am thinking of naming the plugin mqtt_slave) and would only run selected stations. For example if the master has 8 valves connected to it and the slave has another 8 connected to it, as in your case, you would set the master to have 16 stations ( under Options > Station Handling > Extension boards = 1). You would make the irrigation programs for all 16 stations on the master. When a program switches a valve on or off that information would be sent to the slave(s) by the mqtt_zones plugin. If the valve is in the 9 -16 range, the mqtt_slave program would activate the appropriate valve on the slave. The master would continue to display the status of the station as if it was on the master and log the run as usual.
It looks like modifying the mqtt_schedule plugin will be pretty easy. I will work on it in the next few days.
In the mean time you can try installing the mosquitto MQTT broker on one of your Pis. This may take some tinkering to get working properly. I have a blog with some instructions for installing mosquittto:
https://xperimentia.com/2015/08/20/insta...ebsockets/
After I have a chance to test things out I will be able to give you some detailed and simplified instructions for setting up mosquitto.
I recommend a Raspberry Pi 3 to rum both SIP and mosquitto.
There is a handy utility that can help with testing of MQTT:
http://kamilfb.github.io/mqtt-spy/
Dan
You would just need to schedule all the stations for both SIP installations on the "master" SIP which would use the mqtt_zones plugin to broadcast all the zone states each time a station is switched on or off.
The "slave" SIP installation would be running the modified mqtt_schedule plugin (I am thinking of naming the plugin mqtt_slave) and would only run selected stations. For example if the master has 8 valves connected to it and the slave has another 8 connected to it, as in your case, you would set the master to have 16 stations ( under Options > Station Handling > Extension boards = 1). You would make the irrigation programs for all 16 stations on the master. When a program switches a valve on or off that information would be sent to the slave(s) by the mqtt_zones plugin. If the valve is in the 9 -16 range, the mqtt_slave program would activate the appropriate valve on the slave. The master would continue to display the status of the station as if it was on the master and log the run as usual.
It looks like modifying the mqtt_schedule plugin will be pretty easy. I will work on it in the next few days.
In the mean time you can try installing the mosquitto MQTT broker on one of your Pis. This may take some tinkering to get working properly. I have a blog with some instructions for installing mosquittto:
https://xperimentia.com/2015/08/20/insta...ebsockets/
After I have a chance to test things out I will be able to give you some detailed and simplified instructions for setting up mosquitto.
I recommend a Raspberry Pi 3 to rum both SIP and mosquitto.
There is a handy utility that can help with testing of MQTT:
http://kamilfb.github.io/mqtt-spy/
Dan