Interfacing with ViewStar Solar Controller

Hi, I'm trying to interface a BB Black with a ViewStar Solar controller. The controller provides a port that's described as a TTL-232 Rj45 interface.

Can somebody maybe provide some guidance as to how I could do this.


You need to be kind of careful here. The only approach that would work in all cases is an active 3.3V/5V level conversion in both directions. There are some buffer chips out there that would work for this, as well as dedicated level shifter ICs.

Rule #1, of course is that the BBB input should never see more than 3.3V. Rule # 2 is that you need to meet the logic thresholds in both directions.

If the solar controller is really 5V TTL, a simple voltage divider at the BBB input and a straight connection to the controller input could work, but the worry is:

Real 5V TTL output stages on new equipment is getting kind of rare. The vendor may be using “RS232 TTL” as a sloppy way of saying that the interface is at 5V levels and the inversion is opposite from bipolar RS232 – but the interface may be at CMOS output levels and input thresholds.

Why is this a problem? Well, using the BBB output to solar controller input as an example, the guarenteed high from the BBB output is (3.3 - 0.45) 2.85V. This would drive a TTL input with a 2V threshold just fine. BUT, if the solar controller input was CMOS, the input threshold would be (0.7 * 5) 3.5V, so the BBB would not be able to drive it.

If I were working on this interface, I’d look at that “TTL” interface with a jaundiced eye and take the conservative level shifter approach.