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RS485 bus wiring

iprowell

Member
I am starting to wire up my expansion boards onto my Elk M1. I have a M1DBH. On hand I have shielded 18/2, shielded 18/4, unshielded 22/6, 26awg cat5, 24awg cat6, and probably a few other sizes.
 
- Should I wire the M1DBH directly to the M1G with everything, even local expansion boards off the M1DBH? What wire type should I use?
- Should I wire the local expansion boards first and then put the M1DBH at the end in the enclosure so that the M1DBH is only used for remote devices?
- What type of cat5/6 wire is recommended for use with the M1DBH? It seems like 24awg is the largest that is readily available.
 
Thanks!
 

RAL

Senior Member
The DBH makes it easy to add/remove data bus devices by simply plugging/unplugging a RJ45.  But each RJ45 adds another possible failure point to the configuration.  And many people have had trouble with making reliable RJ45 crimps.  Get yourself a good quality crimp tool!. 
 
I would wire up any expansion boards or other devices that are close to the M1 as one branch of the data bus (without the DBH), and then connect the DBH as the second data bus branch to connect the remote devices.
 
I would use Cat5e cable.  The RS485 bus is pretty slow and won’t get any advantage from the extra bandwidth Cat6 provides. Cat6 tends to be stiffer and doesn’t bend quite so easily in tight spaces like the Elk enclosure. 
 
Cat5e cable is usually 23 or 24 AWG, unless you go out of your way to purchase 22 AWG.   22 AWG will be better for carrying power to any remote devices, as it will reduce voltage drop (unless you are using remote aux power supplies).  But with 22 AWG, you will need to be sure to get RJ45 plugs that can handle the heavier wire gauge.
 
If you haven’t done so already, you should do a current draw calculation to make sure that the M1 can power all the devices you plan to connect.  If you have a load of more than 1.0 Amps, you will need to add aux power supplies.   Elk has a spread sheet you can download to make this calculation easy.
 

lanbrown

Active Member
For RJ45 male plugs, look into Panduit FP6X88MTG connectors.  They also support 22 to 26 AWG wire.  They can also be reused around 10 times.  So they are more expensive than crimp style plugs, but they are more versatile.  I already had a lot of Panduit cabling in my network, so I've used these where I needed a male plug without the need to terminate it to a female and then use a patch cord.
 

iprowell

Member
RAL said:
The DBH makes it easy to add/remove data bus devices by simply plugging/unplugging a RJ45.  But each RJ45 adds another possible failure point to the configuration.  And many people have had trouble with making reliable RJ45 crimps.  Get yourself a good quality crimp tool!. 
 
I would wire up any expansion boards or other devices that are close to the M1 as one branch of the data bus (without the DBH), and then connect the DBH as the second data bus branch to connect the remote devices.
 
I would use Cat5e cable.  The RS485 bus is pretty slow and won’t get any advantage from the extra bandwidth Cat6 provides. Cat6 tends to be stiffer and doesn’t bend quite so easily in tight spaces like the Elk enclosure. 
 
Cat5e cable is usually 23 or 24 AWG, unless you go out of your way to purchase 22 AWG.   22 AWG will be better for carrying power to any remote devices, as it will reduce voltage drop (unless you are using remote aux power supplies).  But with 22 AWG, you will need to be sure to get RJ45 plugs that can handle the heavier wire gauge.
 
If you haven’t done so already, you should do a current draw calculation to make sure that the M1 can power all the devices you plan to connect.  If you have a load of more than 1.0 Amps, you will need to add aux power supplies.   Elk has a spread sheet you can download to make this calculation easy.
 
I went through the spreadsheet and am well under the 1 amp limit. Thanks for pointing that out.
 
Right now the only remote items on the RS485 bus will be 2 keypads. I'll give it a go with standard cat5. The 22awg was spendy and going to take a while to arrive. I have dealt with crimping down my share of RJ45 plugs so am familiar with the challenges.
 

RAL

Senior Member
iprowell said:
I went through the spreadsheet and am well under the 1 amp limit. Thanks for pointing that out.
 
Right now the only remote items on the RS485 bus will be 2 keypads. I'll give it a go with standard cat5. The 22awg was spendy and going to take a while to arrive. I have dealt with crimping down my share of RJ45 plugs so am familiar with the challenges.
For just the keypads, the standard Cat5 cable should be fine as long as the cable length is reasonable. 50' of cable would give you about 0.4V voltage drop for a M1KP keypad.  Unlike the data bus signals, the power wires are not daisy chained out and back from one M1DBH jack to the next.  So each of 2 keypads would just see the drop caused by their own cable.  A M1KP2 draws less current and the voltage drop would be about 0.22V. 
 
When purchasing cable, my preference is for pure copper cable.  A lot of Catx cable is Copper Clad Aluminum (CCA).  It costs about half as much, but can cause reliability problems over time. 
 

lanbrown

Active Member
Some of the cable is also stranded rather than solid.  Then you need the correct plugs to use with stranded Catx cable as well.
 

iprowell

Member
The only long run will be for a M1KPAS so even lower draw than the M1KP2. There will be a M1KP near the main unit and I will probably just use 4 wire to that with it terminating the "local" leg of the bus.
 

mikefamig

Senior Member
The connections on the data bus are critical. I learned the hard way that the lower the power of the signal that you put on the wire the more critical the quality of your splice becomes. Any increase in resistance on the data bus can cause problems. I have worked on automotive and house AC wiring for years and it is very forgiving compared to the M1 data bus.
 
when I first installed my M1 I spent months troubleshooting a problem where my system just reported random violations of zones for no known reason. I was calling the female elk voice the crazy lady from North Carolina. It turned out to be a poor splice on the data bus that was causing the problem. I used a bean type crimp connector and it was only by going through all of my splices one by one and re-doing them that I happened across the guilty party.
 
To sum it up, I now believe that the less splices and RJ connectors the better. I did install a DBH and I do believe that it causes problems now and then. The Lady from N Carolina will occasionally have a bout of dimentia, reporting doors opening and such,  and I go around and clean all of the RJ connectors, including the DBH, and the problem goes away for another year or so. Other than that the system is rock solid and I have no regrets.
 
There was a member here that used the name DELinstallations who was a professional installer and he advised me then not to use the DBH and I should have taken his advice. I could remove it and replace it with good soldered splices but it really isn't problem enough for me to bother to do that so I would say it is a judgement call. How does the DBH help your installation? Is it needed? Is it even helpful? I found it to be very helpful by making it easy to plug things in and out during installation and testing but after that it is of no use to me.
 
Mike.
 

iprowell

Member
mikefamig said:
The connections on the data bus are critical. I learned the hard way that the lower the power of the signal that you put on the wire the more critical the quality of your splice becomes. Any increase in resistance on the data bus can cause problems. I have worked on automotive and house AC wiring for years and it is very forgiving compared to the M1 data bus.
 
when I first installed my M1 I spent months troubleshooting a problem where my system just reported random violations of zones for no known reason. I was calling the female elk voice the crazy lady from North Carolina. It turned out to be a poor splice on the data bus that was causing the problem. I used a bean type crimp connector and it was only by going through all of my splices one by one and re-doing them that I happened across the guilty party.
 
To sum it up, I now believe that the less splices and RJ connectors the better. I did install a DBH and I do believe that it causes problems now and then. The Lady from N Carolina will occasionally have a bout of dementia, reporting doors opening and such,  and I go around and clean all of the RJ connectors, including the DBH, and the problem goes away for another year or so. Other than that the system is rock solid and I have no regrets.
 
There was a member here that used the name DEL installations who was a professional installer and he advised me then not to use the DBH and I should have taken his advice. I could remove it and replace it with good soldered splices but it really isn't problem enough for me to bother to do that so I would say it is a judgement call. How does the DBH help your installation? Is it needed? Is it even helpful? I found it to be very helpful by making it easy to plug things in and out during installation and testing but after that it is of no use to me.
 
Mike.
Mike,
Thanks for the input. The intent of the DBH was future flexibility. Nearly everything on the data bus will be in a single cabinet and will be on a single leg of the bus. Right now I only plan to have one keypad off the DBH. Maybe I can just use the DBH for initial testing and keep it at the end of the second bus leg. So far I have it wired for testing with one keypad off of it with no issues, but I will keep in mind that it could introduce issues.
 
Whenever possible I solder things to keep stuff reliable. We had gremlins in our heat pump when it rained until I soldered the control wires. Now everything is 100% reliable.
 
Thanks,
Ian
 
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