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CRS112-8G-4S-IN - DC input/POE-out questions

Posted: Wed Jun 17, 2020 4:44 pm
by stoser
Hello. I am considering purchasing a couple of CRS112-8G-4S-IN to provide power to some equipment over POE-OUT at a remote site.

The web page states that "CRS112-8P-4S-IN can power 802.3af/at devices if 48-57 V DC input is used (unit will automatically detect and provide correct power to devices)". My first question is: If a 48V-57V power supply is used, can some of the ports provide 48V and others 24V? An I interpreting the web page correctly?

I know that with other models that have two DC inputs (DC in and POE in for example), that both can be plugged in at the same, and the higher voltage is used by the unit. In this way an effective yet simple fail-over can be achieved.
My second question is: Can both of the DC inputs on the CRS112-8G-4S-IN be used simultaneously for failover? Specifically I would like to use the 48V power supply plugged into the electrical AC supply, and plug the other DC input into an 18V battery bank. All of the equipment that will plug into the POE OUT ports of this switch functions at 18V, but ideally all of it can use 24V and some can use 48V.

Thanks for any insights

Re: CRS112-8G-4S-IN - DC input/POE-out questions

Posted: Wed Jun 17, 2020 5:10 pm
by mkx
AFAIK RouterBoard devices don't do voltage conversion, hence requirement about proper power source attached.

Meaning that if you use 48V and 18V sorces, PoE out will be either 48V or 18V. When either of them fail, CRS will continue to work but will cut off PoE out on ports where power was provided (selected upon voltage needed) by failed power source.

Re: CRS112-8G-4S-IN - DC input/POE-out questions

Posted: Wed Jun 17, 2020 5:30 pm
by stoser
Many thanks mkx

I have not been able to interact with one of these units, so I am still a little confused. If I interpret your reply correctly, if both the 48 volt supply and the 18 volt supply are simultaneously attached and functioning, the POE outs will ONLY be 48V?

Or is there a way to select between the two input voltages uto supply either 18V or 48V to different ports at the same time?. In my setup I would ideally have 48V POE OUT on one ports, and the other 7 should have 18V. If the 48V supply fails, ideally the single port that had 48volt would either cut off, or automatically drop to 18v.

Re: CRS112-8G-4S-IN - DC input/POE-out questions

Posted: Wed Jun 17, 2020 11:36 pm
by mducharme
Many thanks mkx

I have not been able to interact with one of these units, so I am still a little confused. If I interpret your reply correctly, if both the 48 volt supply and the 18 volt supply are simultaneously attached and functioning, the POE outs will ONLY be 48V?
If you plug in both 24V and 48V AC adapters into the device, then it can power 24V and 48V devices simultaneously on different ports. If either adapter fails or comes unplugged, the switch itself will still have power, but any devices that use that voltage that came unplugged will lose all power and not come back until the adapter is replaced or plugged back in. So the dual supplies really only provides redundancy for the switch itself (the switch will stay running even if either fails), but any devices that use the voltage that that power supply generated will be offline.

Re: CRS112-8G-4S-IN - DC input/POE-out questions

Posted: Thu Jun 18, 2020 5:09 am
by stoser
Thanks mducharme, your reply helps me to understand the way the redundancy works.

Re: CRS112-8G-4S-IN - DC input/POE-out questions

Posted: Thu Jun 18, 2020 9:54 am
by Elans
@mducharme answered on most of your questions. Currently there are only 2 available devices with voltage conversation on board using single power supply - CRS328-24P-4S+RM and CRS354-48P-4S+2Q+RM.
For other devices it is like "what you give is what you get", therefore, as example, if you will use 24V to power the device, then on poe-out ports you will have 24V ( - possible voltage drop due to cable length, quality etc.)

Re: CRS112-8G-4S-IN - DC input/POE-out questions

Posted: Thu Jul 09, 2020 3:01 pm
by dunkercz
Hello, we are solving problem with +/- DC voltage. CRS112 is grounded to negative pole of 48V DC (ie. +48V power system). Telecommunication standard is -48V power system (grouded positive pole of 48V DC).
There si problem with HW (Dragon Wave, Ceragon, ATH, Ericsson), which he usec -48V. If we connect CRS112 (+48V) a short circuit occurs.
It is possible to optionally adjust the CRS112 to be selected -48V or +48V power system or do not ground the power supply?
Thanks for answer.

Martin