Reef Angel Forum

What is the maximum number of expansion hubs that can be connected to one RA+ controller? If applicable, what is the preferred topography with multiple hubs, ie: star, daisy chain?

just 1.

If you don't mind me asking, is the limitation software or hardware?

Hardware.
I need to create a hub with more ports, but did not find the time yet.
But if you really want to add more, you can also splice 2 usb cables together in parallel.
In fact ports 9 and 10 of the hub are like that.
It should work. I've never tested, but each port in the hub should be capable of handling 2 expansion modules just fine.
Such USB cable is not commercially available because the USB standard doesn't support it, but we are not using the USB protocol anyway.

They do make usb y cables but they may only have wiring for power...it might be worth seeing what can be sourced...

rimai wrote:Hardware.
I need to create a hub with more ports, but did not find the time yet.
But if you really want to add more, you can also splice 2 usb cables together in parallel.
In fact ports 9 and 10 of the hub are like that.
It should work. I've never tested, but each port in the hub should be capable of handling 2 expansion modules just fine.
Such USB cable is not commercially available because the USB standard doesn't support it, but we are not using the USB protocol anyway.

More ports on an [expansion] Hub would be nice so the tip on splicing/paralleling USB cables is good news!

I am also thinking of a large system with a couple dense groups of sensors/inputs that are spaced apart from one another, where having a number of normal Hubs might be easier to wire/configure. From a brief look at the datasheet and some NXP application notes (AN255 Fig. 26), can we use the expansion-I2C pins on the PCA9518 to add a new 'port' on the Hub that allows for connection of multiple Hubs?

I thought about that, but decided to leave it alone.
The current hub uses 2x PCA9518 and uses those expansion pins to talk to one another.
That's why it only has 9 ports and the 10th is just a parallel of the 9th port.
I'm probably going to use 4x PAC9518 for a total of 20 ports on a bigger version of the hub.

Have you breadboard-ed or otherwise tested multiple PCA9518's with a longer distance on the expansion pin bus?

No, that's why I decided to leave it alone

Ah, okay. Well, I might pick up an Arduino and do some testing then.

Thanks for the help, Roberto!