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RS485 Optical Isolation for M1 Bus

kwschumm

Active Member
I've got a shop 150 feet away from a house that will be connected to the M1 via underground conduit.
 
Previously I've worked with RS485 systems that have been fried due to ground potential differences between buildings.
 
So I'd like to install an optical isolator in the M1 bus between buildings.
 
Has anyone done this?
 
I'm looking at using the B&B 485OP optical isolator but am not sure how to establish the ground reference signal between buildings with the M1 bus.
 
https://buy.advantech-bb.com/Serial/Isolators-and-Repeaters-RS-232-422-485/model-BB-485OP.htm
 
TIA for any help.
 

RAL

Senior Member
kwschumm said:
I've got a shop 150 feet away from a house that will be connected to the M1 via underground conduit.
 
Previously I've worked with RS485 systems that have been fried due to ground potential differences between buildings.
 
So I'd like to install an optical isolator in the M1 bus between buildings.
 
Has anyone done this?
 
I'm looking at using the B&B 485OP optical isolator but am not sure how to establish the ground reference signal between buildings with the M1 bus.
 
https://buy.advantech-bb.com/Serial/Isolators-and-Repeaters-RS-232-422-485/model-BB-485OP.htm
 
TIA for any help.
This question has come up before in relation to a fiber extension.  Some people have tried it and failed.  It comes down to a problem with how quickly the optical extenders can switch from transmit to receive mode and how that affects latency on the M1 bus.  If things don't happen quickly enough, it won't work.
 
The optical isolator you linked to has separate receiver and transmitter interfaces using 2 pairs of wires.  But the Elk bus uses a single pair for both.  The two pairs can be tied together, but then I think you will get into the same latency issue with the turn around time on the bus when it switches from transmit to receive.
 
I have a vague recollection that someone found an optical extender that worked for them.  But I haven't found that thread.
 
Here is another thread that has some discussion of the issues. 
 
http://cocoontech.com/forums/topic/30396-elk-comm-buss-over-fiber/
 

kwschumm

Active Member
Thanks for the information.
 
That turnaround time is too slow is surprising. This RS585 isolator specs a turnaround time of 260ns at 38.4kbaud. On the system I coded for the turnaround time was around 19ms at 19.2kbaud but that was ten years ago and things are faster now.
 
B&B has had several revisions of the isolator so maybe it will work with the latest. They also have a statement in the quick start guide that if we need a specific turnaround time to give them a call.
 
My plan is to get the system up and running without the isolator and then give it a try and hook up a scope if it fails. We won't move in for a few months but I'll try to remember to post back with results.
 

sic0048

Senior Member
What devices are you trying to use?  I don't think ELK has a wireless keypad (although you can use the mobile apps to arm and disarm), but at least you could look at wireless sensors.  
 

kwschumm

Active Member
The shop is pretty well wired. Door sensors, motion detectors, water leak detectors, CO/Heat detectors, glass breaks, interior sounder, exterior strobes.
 
I'm using an Elk-P212S supervised power supply, one input expander, one output expander and a keypad in the shop. M1 bus will run underground to the shop.
 

kurtmccaslin

Active Member
I have the same setup-- except with a HAI Omnipro II.   I got hit by lightning twice and eventually went to fiber between the house and shop.   I bought SerialComm SER-FIBER-MM for both ends and it worked plug and play.   I had some concern about latency on the bus, but I finally decided that the cost of trying something was better than getting hit by lightning again.  (I am on a hilltop)
 
I worried about the ground reference too.   I finally decided that you don't need the ground reference.   If you are running copper, then the ground reference is critical.   For fiber, each converter on both ends establishes its own ground reference.   No need to tie them together.
 
Your optical isolator should work, however, I don't know if 2kV isolation will protect you from lightning.

Also, if you power the isolator from your P212S power supply, you will have a nice battery backup. It will reduce false alarms when you lose power.
 

kwschumm

Active Member
rockinarmadillo said:
I have the same setup-- except with a HAI Omnipro II.   I got hit by lightning twice and eventually went to fiber between the house and shop.   I bought SerialComm SER-FIBER-MM for both ends and it worked plug and play.   I had some concern about latency on the bus, but I finally decided that the cost of trying something was better than getting hit by lightning again.  (I am on a hilltop)
 
I worried about the ground reference too.   I finally decided that you don't need the ground reference.   If you are running copper, then the ground reference is critical.   For fiber, each converter on both ends establishes its own ground reference.   No need to tie them together.
 
Your optical isolator should work, however, I don't know if 2kV isolation will protect you from lightning.

Also, if you power the isolator from your P212S power supply, you will have a nice battery backup. It will reduce false alarms when you lose power.
I believe you are exactly right. I hadn't thought about powering the isolators off the panels, thanks for the idea. there is still time to run another wire for that before the sheetrock goes up. In this case the shop isolator is in a separate can from the P212S.
 

RAL

Senior Member
My choice would be to power the optical devices off of a separate power supply, with battery back up if necessary.  By supplying power from the alarm panel, you'd be providing a path right back to the thing you are trying to protect, should the optoisolator fail due to too high a surge.
 

kwschumm

Active Member
RAL said:
My choice would be to power the optical devices off of a separate power supply, with battery back up if necessary.  By supplying power from the alarm panel, you'd be providing a path right back to the thing you are trying to protect, should the optoisolator fail due to too high a surge.
I'm driving a lot of stuff with an external Altronix supply. Pretty much everything except closed contact inputs and outputs.
 
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