Cbus switches not working

Any advice appreciated. I have a client who has reported that the light switches are taking two or three presses before a light comes on. Been and had a look, nothing obvious and we fitted a burden which briefly (two weeks) seemed make a difference. There are 2 x 8 chan dimmers WP and 1 x 12 chan relay WP. Keypads are all reflection range. Installed and worked no probs for approx 15 years. Voltages on network are fine. Thanks for any advice

 

My initial go-to (and quick answer, sorry!!), would be to check the C-Bus with an Oscilloscope, not a DMM. I have had a flaky power-supply before, which wasn't really registered by the DMM (or the unit voltage reports). But the voltage fluctuations were very obvious on an oscilliscope, and they were very brief (Probably messed with the clock signal)?

In my situation a repower of the CBUS power supplies seemed to make it come right, for a short while, then it would start again. Then comes the fun part.... Figuring which power supply on the bus is causing this issue (if that's what you are seeing too). Just needed to connect a few input units individually to each Output unit with a power supply (to give it some load), and monitor with an oscilliscope... The unit with the dodgy power supply made itself known pretty quickly. My install was about 12-13 years old when that power supply issue manifested itself.

That was a few years ago now, and has been right since that Output unit (with bad PS) was replaced, but I do have my suspicions that I will be going through this exercise myself again soon, with very rare/occasional non-responsive button pushes, although not at the level where I am yet motivated to diagnose it yet.

 

Also check the voltage on the + and - line of the cbus with respect to earth ground. THEY MUST MATCH.

 

Great reply. I have been maintaining and diagnosing C-Bus for 20 years. It you don't have a 'scope, I would be looking for a unit with a built in power supply as the culprit like this:- Try disabling the clocks on the PSU units and disconnect the C-Bus, is the C-Bus LED flashes a bit it is faulty. Try again enabling the clocks on the PSU units and removing the C-Bus cabling. if the C-Bus LED flashes that unit is faulty. You can often stabilise the system by adding a 350ma PSU, which takes the pressure off of the built in units. Doing this will prolong the life of units that are working but old. it keeps them cooler and working less hard. The 350 PSU seems to supply in advance of the built in units. Also, keep the units as cool as possible, plenty of air, they will last longer. Make sure the vents protrude through the din rail enclosure front panel. have air flow via the enclosure too. if they throw a real wobbly, turn them off for 20 mins. When they come back on they will work for a while, you can recover programming, etc.

Units where the LEDs seem dull are old and the PSUs are weak. Also if the buttons on the unit are tempermental.

Get a separate PSU, best option for extended life. Replace units with no PSU with a unit with a PSU if the die if you have no extra space. The max amperage warning can be taken with a pinch of salt. So long as the units that supply power and that use power are distributed evenly on the bus, and no one part of cable carries anywhere near the the max allowed, having more power available isn't a problem. They only use what they need, not more. Just dont overload the cable.

Also, check the impedance. Don't be too low. One burden is usually enough, and a dedicated hardware one is better than a software selected internal unit one. It isn't affected by unit issues.

 

This is bad advice. If you short the bus the full current will flow though that section of the cable. Also, too many power suppllies will upset the bus impedance.

If the system isn't working, going outside of spec isn't likely to fix it.

One burden is always enough. Large systems may need no burden.

 

Ashley, you are taking it to extremes to prove a point and cherry picking my advice. I never said add PSUs if it isn't working, but it can get it working to extract code. I gave advice on how to diagnose and fix, then how to extend life. There is no harm if you have extra power on a well designed system. And before you go extreme, I am not saying to have triple power.

If you short the bus the PSU will not deliver max current, they set back until the short is removed. Besides, even if a balanced system was shorted, in your scenario the current could flow in both directions towards the break, making overload less likely. All PSUs would have to be at one end and the short at the other, even then the short would drop the voltage quickly, and reduce the current. However the PSUs would back off, well within the adiabatic envelope of the cable so no overheat.

How many is too many PSUs? How much do they affect the impedance? Does a PSU affect it more than 10 more e-DLTs? The system works within a huge range of impedances. It depends on the topology of the system, the cable tree, the placement of loads and PSUs. The voltages vary at every point in the bus. A well designed and balanced system will have less voltage variance. An extra PSU in the correct spot can stabilise an aging system.

One Burden is enough, which type? Funny but a couple out of the 100s I maintain need 2 for stability, noisey places, lots of RF, 2 very specifically placed hardware burdens to tie down noise. You can see it clean up on the 'scope and come back if removed. The C-Bus signal degrades the further it moves from the burden. The resistance increases the further from a burden. Don't read the leaflet at me, there are exceptions for every rule.