RPS Basics


What is an RPS?

RPS stands for Rack Protection System.  Each VME rack has one, as does the DAQ rack.  Its job is to cut power to the rack in event of a problem such as fire, over voltage, or lack of cooling air flow.


The 1U tall black module at the top front of the rack is the RPS.  The 3U black module in the lower back of the rack is the AC Relay Box.  The RPS has many sense wires monitoring things in the rack, see the online documentation for details of what gets watched and how. 


RPS Buttons and Lights

There are 5 buttons and 3 lights on the front panel.

Buttons, from left:

1.     Smoke Detector Reset

2.     VAC Reset

3.     Output Latch Reset

4.     Clear Display

5.     Alarm Silence (to the right of the lights)


Lights, from left:

1.     Red ALARM light

2.     Yellow WARNING light

3.     Green OK light


Meaning of Lights:

If Green light is on, things are currently OK.

If Green light is out, things are currently NOT OK.

If Red or Yellow lights are on, there has been an ALARM or WARNING since the last time someone cleared the display.

(so you could have all three on at once – it’s ok now, there used to be problems, but things haven’t been cleared)

ALARMS are things bad enough to cut the power (like a smoke alarm).  WARNINGS are things we should know about but which won’t hurt the electronics (yet) such as DC voltages too low.



What the Buttons Do:

1.     Smoke Detector Reset

·       Resets the smoke detector if there was a real smoke alarm.

·       Triggers a test smoke alarm if things are ok.  Useful to force an alarm to cut the power.

2.     VAC Reset

·       Push this button to force the RPS to allow AC power to pass.

·       Hold it down to start a rack that’s been off.  This allows things like the fans time to get revved up in order to make the RPS happy so it will let the power stay on by itself.

3.     Output Latch Reset

·       Clears the RPS’ internal memory of past problems.  If this is not pressed after problems are resolved, the computer will think there’s still a problem.

4.     Clear Display

·       Clears the display.  Useful to get rid of old lights.  If there’s still a problem, they’ll come right back on.

·       Does not clear the computer readout of an old problem.

5.     Alarm Silence

·       Shuts up the annoying buzzer.  Does nothing else.  For Temporary Sanity Saving only.  The silence is undone the next time you hit “Output Latch Reset”


AC Relay Box Buttons, Lights, and Switches:

The AC relay box in the back takes power from the wall, a 208V/20A circuit and a 120V/20A circuit.  Everything else in the rack plugs into this box (except the RPS itself, which plugs into wall 120V).


There is an overall “DC Control” light.  If this middle light is green, the RPS is telling the relay box to please let power pass.


Each of the two circuits has a 20A breaker (on this relay box as well as on the wall!) and two lights.  The “Input” light is lit if that circuit is plugged into the wall and receiving power.  The “Output” light is lit if the power is being passed to the sockets on the back of the relay box.





Procedure to power down a rack:


NOTE: if there’s going to be a power outage or if it looks like it’s going to last a while, please gracefully shut down the racks and cut their power at the main breaker box, to avoid power flickers!


1.     Make sure there’s not a run going on involving this chunk of electronics.

2.     Press the “Smoke Detector Reset” button.  This forces an ALARM condition and cuts power to the rack.

3.     Power is now off.  Consider turning off the breakers on the relay box and unplugging the relay box from the wall for safety if you’ll be messing with power wiring.

4.     Note that the RPS is still on at this point!  That’s a good thing in general, but consider also unplugging the RPS if:

a.     You will plug or unplug any sensors into the RPS.  Fuses can blow if you plug or unplug sensors while the RPS is powered up (little green picco fuses inside the RPS chassis).

b.    Wall power will be going up and down in a bad way.  Easier to shut things down at the main wall breaker box though.


NOTE: If all you want to do is to work on the VME crate or FE electronics, you can turn the WEINER power supply and/or the Harvard PDB off without powering down the rack.  The RPS will sound and there will be a yellow WARNING.  But, WARNINGs don’t shut the power to the rest of the rack, keeping more things happy.















Procedure to power up a rack:


NOTE: if power comes back up all over, say after a power outage, then some racks will power up on their own and others won’t.  This is because some racks can get all the way back up in the few seconds power-on grace period the RPS gives.  Others won’t, especially if the RPS has remained powered and warmed up.  So, many racks might get to step 5 or 6 all on their own.  Saves you button pushing.


1.     Make sure things are safely plugged in and ready to get AC power.

2.     Check that the AC relay box is plugged into the wall on both circuits, and that the relay box breakers are “On”

3.     Make sure the RPS is plugged in at the wall

·        A newly plugged in RPS will scream, as the stuff it’s monitoring won’t be happy because it will be off!

4.     Hold down the “VAC Reset” button till the green “OK” light comes on.  This might take the better part of a minute – worry if it takes more than a minute, and go see what the problem might be.

5.     Sometimes, the WEINER power supply does not come on when power is applied.  They are supposed to do so, but sometimes forget.  In this case, flip the little red front panel power switch up while applying AC power, and you should see it start to boot.

6.     Once the green “OK” light is on, reset the readout and display:

a.     Hit the “Output Latch Reset” button

b.    Hit the “Clear Display” button

7.     At this point, only the green light should be on and everything in the rack will be humming along.













Online Status Readout


The first place to look is the DCS status webpage.  This shows all problems with the DCS subsystems and decodes the current RPS status if there is a problem (login as numi, standard password):




The iFix map of MINOS on the DCS windows machine in the control room will be back up eventually.  Problem areas flash on a map of MINOS and can be clicked on to find out more and/or control things.


Lastly, take a look at the /home/minos/rps/rpsdaemon.log file on the dcsdcp linux machine.  This file is a running text dump of RPS problems.  If you see many problems on the status page, you can page through this text file and see what problem might have started things shutting down – once power to the rack is off, you of course have many problems all at once!


More Documentation


Documents online go into gory detail about most of this stuff if you need more information.  Look at the UMD MINOS homepage (the home page for the browsers on the DCS Windows machine in the control room) and click on Docs, or go to it directly:




People to Call with Problems or Questions:


Alec Habig, 218-726-7214 (UMD) 218-724-9158 (home), ahabig@umn.edu

Jason Koskinen, 218-726-8395 (UMD Lab), koskinen@alum.rpi.edu


Brian Bock is also in that lab and might be able to help.