Elk M1XSP and the RS485 Bus

[ultrajones]

Hello. I am looking to install a second Elk M1XSP to my Elk M1G in order to add Insteon support. I already have one Elk M1XSP installed connected to my thermostat. The RS-485 Data Bus currently has two legs. One going to a keypad and the other going to an Elk M1XSP. Can I simply daisy-chain the RS-485 Data Bus cable from one M1XSP to the other? If so, how? The manual was very confusing and didn't show any examples of how to "daisy-chain" them together.

Thanks in advance for any help you can provide.

Regards,
Ultrajones

 

[rfdesq]

I think it may be easier to use an ELK data bus hub.

 

[slipnfall]

How are your existing two RS485 devices wired currently, and do they have terminating resistors installed?

If both are wired directly to the M1G(and both terminated), you can 'extend' the existing XSP chain by wiring the second one in parallel(AT the original XSP, not the M1G). Then move the chains terminating resistor to the new device (end of the chain)

In the end everything is wired in parallel, but you have to wire the next device to the last, and move the resistor to the 'end'.

Clear as mud?

-----------------------------

PS for the interested: I see that the ELK documentation shows you can wire two device chains on the RS485 bus... If you've got a long length of keypad cable, then a short cable for a XSP (both wired directly to M1G), this will cause impedance mismatches and consequently introduce signal reflections, no? I thought the EIA485 only spec'd a transmitter to drive one twisted pair.

 

[rocco]

Welcome to Cocoontech, Ultra. It feels strange to see UltraJones listed as a "Newbie".

I thought the EIA485 only spec'd a transmitter to drive one twisted pair.

Yes, you are right, but I believe it really is only one twisted pair.

The transmitter does not have to be at the end of the chain, but can be anywhere along the chain. Matter of fact, every device on the RS485 bus is a transmitter.

So I think that each twisted pair is actually a portion of the same chain, and the Elk is actually in the middle of the chain. That is why there are only two terminating resistors, one at the end of each run of wire, and no terminators on the Elk itself.

 

[Steve]

If both are wired directly to the M1G(and both terminated), you can 'extend' the existing XSP chain by wiring the second one in parallel(AT the original XSP, not the M1G). Then move the chains terminating resistor to the new device (end of the chain)

In the end everything is wired in parallel, but you have to wire the next device to the last, and move the resistor to the 'end'.

Don't you mean 'series' instead of parallel? I always thought when devices were connected in a chain it was series and if the were 'homerun' side by side to the controller it was parallel. In fact, the M1DBHR lets you create up to 8 'parallel' branches which can then have devices run in series.

As long as you have a terminating resistor on each end of the bus/chain you can chain many devices together. So, as described, you should have a terminating resistor on the current XSP, either install the new one after it and move the resistor, or insert the new one in front of it. Wire goes from M1 to first XSP and then from that XSP to the next one so you will wind up with 8 wires (2 for each conductor of the bus) on the first XSP and 4 on the last one. Just make sure the resistor is on the XSP with only the 4 wires on it.

Hope that helped a little.

 

[slipnfall]

Ultra: don't let us run you off: let us know how you currently have your keypad and XSP wired and we can tell you exactly how you need to integrate a second. It's not hard at all.

So I think that each twisted pair is actually a portion of the same chain, and the Elk is actually in the middle of the chain. That is why there are only two terminating resistors, one at the end of each run of wire, and no terminators on the Elk itself.

This makes sense: technically every device, including the M1G is a transciever, capable of taking control of the bus. If I'm not mistaken, the M1G has a terminating resistor on the panel, which is used if the M1G is at the 'end' of this databus. Otherwise it is in the middle as you say.

Don't you mean 'series' instead of parallel? I always thought when devices were connected in a chain it was series and if the were 'homerun' side by side to the controller it was parallel. In fact, the M1DBHR lets you create up to 8 'parallel' branches which can then have devices run in series.

I think we're on the same page here, just a difference in 'verbage. Electrically every single RS485 device is wired in parallel to the +,-,A,B terminals. The devices themselves however are in a logical series. Think about the wiring at a keypad when you use the databus hub: the 'incoming' A/B is paralleled to the outgoing A/B. The devices are in series, but the wiring is in parallel.

Thanks for your patience: I obviously don't have any hands-on 'yet with the Elk, but have a habit of trying to understand the nitty gritty.

Cheers,
Jamie

 

[acdcelectric]

Steve,

Series refers to the fact that the etire signal/power runs through the first device and then to the second, third etc, with each device's internal load dropping the voltage based on resistances. In a series circuit all of the current from each device flows through the one ahead of it as well, making current flow highest in the first device in line.

A parallel circuit "taps" each device off of the wiring without having the other device's loads flowing thorough it. In that way, voltage remains constant and the current that esch device draws is only carried by the wiring harness.

This is how the electrical outlets in your house are wired.

The "chain " analogy" can be misleading iunless you think of the 485 bus as the chain and ech device as a charm on a bracelet...LOL.

 

[Steve]

Sorry, did not mean to confuse even more
Yes, electrically it is parallel since all the same wires are connected together (+ to +, - to -, A to A, B to B ). I guess I was trying to describe it more the way I think about it and made it worse. But I think we are all on the same page, sorry for the confusion.

 

[slipnfall]

Yeah, 'chain' was a bad analogy I suppose. Thanks for clearing that up.

I'm sure there's a ton of other threads on this: no need to rehash, but thanks for following along.

This begs to question if the industry standard wireless RS485 devices could be implemented.

 

[ultrajones]

Thank you to all that replied. I am still confused by the manual referencing the 120 ohm resistor. Is that automatically enabled between the A & B data lines when I terminate a device?

 

[WayneW]

Yup

 

[slipnfall]

Yes: the resistor is on-board. Installing that jumper puts the resistor into the circuit.