low latency trigger advice needed

I am trying to put together a sensor network with a 500m radius.
One base unit with 4-6 sensor nodes.
This is all straight forward, but I need some advice on getting trigger events to base in as little time as possible and would like to avoid typical pitfalls when it comes to latency.

I currently have Pro 900HP modules, but I think the Series 1 802.15.4 modules would be a cheaper solution.
The amount of data would be minimal as I only need to communicate a triggered event.

Any advice on product selection and or methodology welcome.

Many thanks

What do you mean by Trigger event? Are you looking at sending UART data when an event occurs from a sensor or do you need to trigger an IO line state change?

I only need an IO line state change.

Then it would be best to use the XBee 802.15.4 modules as they offer DIO line passing and the others don’t. See http://knowledge.digi.com/articles/Knowledge_Base_Article/XBee-802-15-4-Digital-Input-Output-Line-Passing/?q=DIO+line+passing&l=en_US&fs=Search&pn=1

That seems to be what I am after. Thank you I will dig into the details.
One question jumps out though, line passing seems to be one-to-one and I am unable to tell with what I’ve seen in the article if you could do it with star networks.
Does this mean that you would set up each node to pass a different pin state to the base?

I found a very useful instructional video by you guys at
(for other readers benefit)

Can you comment on a suitable sample rate (state change) that would help with the unit reporting state changes as soon as possible. I understand that the sample rate increments in 1ms, and my first thought would be to set it to the lowest value possible, what do you think?

Much appreciated.

You can configure the modules for a point to point as you have noted and as a Point to multi-point connection with one pin from many modules or from many to one pin or from one pin to one pin form many different modules.

I believe that the additional data you are looking for with respect to sample rates can be found at http://knowledge.digi.com/articles/Knowledge_Base_Article/Digital-and-analog-sampling-using-XBee-radios/?q=DIO+line+passing&l=en_US&fs=Search&pn=1.