RCS TR-40 and Outside Sensor?


Active Member
Going over the documentation to the TR-40 I'd like to add the outside temperature sensor but it suggests I need to physically cable it from the Wall Display Unit (WDU).

As this was a retrofit, I don't have enough free pairs to run it from my WDU. Can I cable it any other way? Is this outside sensor a RS-485 device? Could it therefore be attached to the RCS controller instead of the WDU?

Anyone have a configuration like this?
I don't know if you can cable direct to the controller. I kinda doubt it.
I don't think the temp sensor is RS485, but very likely just a simple resistance change device.

You could also use a 1-wire temp device and a serial or USB 1-wire to PC adapter and the mcsTemperature plugin (if you use Homeseer).
Just got a note back form RCS:

Two ways to do OT for your RS485 network:
1.) use the RCS OS21 outdoor sensor and wire it [ 4 wires ] direct to an
RS485 network connection [ like on the TR40's TCU ] and take in the OT
to the network controller which then sends it to the TR40...we recommend
this configuration;
2.) use an RS15 [ indoor remote ] sensor, weatherize it and set it up
for OT collection [ 4 wires to the TR40's WDU remote sensor terminal
block ] ; the OT's now available immediately at the thermostat, but will
have to be collected by the network controller. This configuration
leaves the thermostat susceptable to lightning damage should the outdoor
unit get hit.
RCS, Inc

Will try this out and see how it goes.
Thanx for posting the update. I didn't know they also have an RS485 sensor. Good to know. Now I don't know whether to get temps into the ELk M1 directly, or via the RS485 you post, or 1-wire!
The outside sensor on the TR-40 is not RS-485, but a Dallas 2 wire interface. All it is is a DS1721 chip in a box. In fact I made my own external sensor for about $5, by getting a DS1721 as a sample from Dallas and soldering it to a surface mount breakout board (the DS1721 is a tiny 8 pin surface mount chip). Then I had to play with the addressing pins to figure out that the outside sensor needed to be set to address 4. Lower addresses are considered inside sensors and their readings are actually averaged (or so it seems) with the sensor built into the WDU. Ther 4 wire terminals in the TR-40 WDU are ground, +5v, data and clock. These are on the same 2 wire "bus" as the built-in temperature sensor.

So you're saying I can use the DS1721, as is, and connect it to the WDU interface with a device address of 4 (and of course mount it in a weather proof box).

But it is required to be connected to the WDU sensor interface. Can I connect it to the RS-485 bus at the TCU box instead of at the sensor terminals in the WDU interface? I'm assuming not.

My senario is that I don't have enough free/spare conductors from my WDU to TCU run, in the orignal thermostat wire, to put the sensor anywhere else without pulling another cable to the WDU location which is troublesome. What caught my eye about the OS21 was that it was connected to the RS-485 bus at the TCU and therefore solved my conductor shortcomings.

Let me take apart the thermostat again and see how many conductors I have left. I'm guessing I only need two (clock and data) because I could get GND and +5 from anywhere.

Thanks - this was a great tidbit/idea!

Just to confirm:

The Dallas chip you suggested:


would only be used from the WDU sensor terminals. So the design and circutry in the OS21 is different in that it has something else supporting the RS-485 communications.

Maybe something like this:


I guess I need to try and understand the TR-40 communication protocol to the RS-485 sensors. The protocol would be standardized but I wonder if/how the query works...

Wanted to keep reporting on my on-going saga with the OS21 and my TR-40.

After purchasing the $117 OS21 sensor, it doesn't seem it will do anything for me - bummer.

It appears that while I can connect the OS21 to the RS485 port on my TCU (TR40 Control Unit), that was recommened by RCS above, that doesn't get me anything. Neither the TCU, nor the WDU, can "read" the information from the sensor. I need something else, some sort of RS485 controller, that will "read" the sensor temperature and "send" it to the TCU/WDU over the RS485 bus. So I'm not sure why RCS suggested it could be connected to the TR40 TCU directly.

In a way I kinda feel misled by RCS. And one would think that for a $117 sensor it would do something by itself as part of the RCS family. I won't be purchasing a "network controller" to make this all work.

Lesson Learned

Thank goodness I bought it from AO who will accept the unused return.

So for anyone reading this thread - take Guy's advice. Build your own sensor (cheaper) and connect it to the WDU. Even if you need to fish cables, it will make your life a lot easier.
I realize this question is from years back, but I am wanting to move forward with adding an external sensor in the manner that Guy proposed (cheap way). Has anyone out there done this?

Basically I am wanting to know about how to set the address one the DS1721 SOIC?

For clarifcation, the wire has to be run to the Wall Display Unit and cannot be run to the Control Unit, is this correct?