OK, I was thinking maybe I can use the feedback LED output of my VMB8PBU as a handy output.
The Official Velbus FAQ for this feedback LED connection looks like this.
I had thought I would just use a (unused) PB# input pin as an output… ie. to drive another logic level input on my own IO module. But it seems is not that simple. Note on the schematic the LED is reversed.
When the LED is ON the PB connection (VMB8PBU input) is driven low-ish… it is at 2.9V, and the C (common) line is NOT ground, it is 3.15V
When the LED is OFF the C is 0.007V and the PB is 4.9V
What officially is going on here? Clearly the C is just another output… but it is not specific to one PB channel. And the PB channel appears to only go between 2.9V for LED ON and 4.9V for OFF.
I am not currently seeing how to make best use of this (as an output).
Thanks
The output signal of this module comes from the electronic bus itself and the signal is not continuous on the bus but multiplexed. That’s the reason you can’t have larger cables
Oh, of course. It is multiplexed. Checking DC voltage levels was the wrong way to go.
I should have checked with the oscilloscope right at the start.
Both ports (C and PB) appear to change roles approx 76x a second.
Well, I think “there goes my hope” to use spare PB ports as logic-level outputs to drive pushbuttons on a garage remote control (Hormann Supramatic 4 BiSecur remote).
I did consider trying to drive an Optoisolator along the same lines as driving a feedback led.
It’s totally possible to configure all the VMBxPB modules a led feedback as followers of button events or relays.
All that stopped me from exploring this further was my understanding of how to drive the optical (emitter) part of an Optoisolator. The NP junction side was easy enough.
That is pretty much what I had been thinking. The remote-control input can be driven from a logic level output pretty easily. I could have easily powered the remote from the VMB8PBU also (it normally just runs from 3V from batteries). This would have allowed me to open either of the garage doors from Velbus without an extra box + bus connection.
However the PB input switches between being an input, and being an output. The common line also switches between being output high and output low.
So for example, when the feedback LED is meant to be ON, the PB input spends 10ms at 0V, followed by 3.3ms at 4.5V.
And the COMMON connection (an output), at the same time, spends 10ms at 4.0V followed by 3.3ms at 0V.
The 10ms part is driving the LED, and the other 3.3ms is presumably with PB as an input.
When the LED is meant to be OFF, the PB input spends almost all of it’s time at 4.5V presumably as an INPUT with a pullup, and the COMMON output spends all it’s time at 0V. I say ALMOST ALL because the do both still spend about 20 microseconds with PB at 2.1V and C at 4.0V… I have no idea what that is for.
So both lines always just look like squarewaves or pulses… there is no steady ON or OFF.
I think that is as far as I got with the VMB8PBU, but not in as much detail.
The VMB6PBN is easier to work with.
LED/ feedback line to Bus 0v is a constant and I’ve got some driving customers Velux KLF200 controllers via TTL relay units.
VMB4PD is supposed to be able to swap modes to become a VMB4IO type device but I could never get it to drive a relay module.
I can send you one if you want to test it?
This is my main velbus test-rig. The modules on the rail get swapped out from time to time but the VMB4PD is actually one of my oldest velbus parts, and it is always there. I just never used one in an install
And sadly it will not fit in the pattress behind my garage light switch
Yes, I now there was a special custom case for it.
Much as I love the VMB4PD, I don’t think it will look right on the wall in the garage.
I shall find another way.
I was not worried about this at all until a guy repairing metal fences at the house recently called me (when I was out) and asked if I would open the garage (like everyone can do that) so he could power his welder. Now I realise that I should be able to do that
