Tstat8 Wiring Topology

I will be trialing a few of the Temco Bacnet/Modbus controllers and programmable thermostats for commercial HVAC applications. Tstats8’s may be used for Fancoil and VAV Box Control and Monitoring over Bacnet MS/TP. The T3-XB’s with T3-IO’s may reside on TCP/IP Ethernet Networks, for AHU Control, Monitoring and Trending of (All) connected Analog Inputs.

My typical (Standard) RS485 bus topology supports 32 (Daisy Chained) devices between “EIA-485 Repeaters” on a single bus of approximately 1200 meters length with 18 AWG twisted shielded cable, 3 repeaters increase the total to 128 devices with a maximum RS485 bus length of approximately 2800 meters (3 repeaters with four segments).

My questions are;

  1. What is the recommended topology for Temco Tstat7’s, Tstat8’s, Tstat9’s, Tstat10’s to T3-XB, T3-Nano or USB-RS485 Gateways/ Controllers/Converters
  2. Are separate end of line termination resistors (EOL, 120 Ohm, 1/4, watt 5%) required on the 485 Bus.
  3. What are the recommended maximum quantity of Tstat8’s per T3-XB’s and T3-Nano’s.
  4. What are the maximum quantity of T3-XB’s, T3-IO’s and T3-Nano’s per each Ethernet Bus

I would like to create a similar wiring topology diagram (for my use), but have not seen / found any info on “Quantity of devices” in the Temco Controls Documentation. Thanks.

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Will have to ask my team to look up the details but in general we have sites with hundreds of devices per 485 bus. I usually don’t put more than 50 devices or so to keep the cabling more manageable. If you run into trouble you can always add a repeater, the OR485 and t3-nano both can operate as optically isolated repeaters.

Termination resistors are optional.

Will add more details from the rs485 chip data sheets shortly, specifically the device loading info.

Thanks for the prompt reply.
I received my trial Temco tstats and controllers today, thanks.

I am setting up a test / demo board with the devices and sensors. The info would be valuable for submittal data if the client initially accepts the lack of UL and FCC listings (UL 916 and FCC Part 15). Bacnet / Modbus is the selling point due to existing Honeywell (Legacy), Siemens, Johnson (Legacy) and recently KMC DDC Bacnet devices. Looking forward to any additional info.

Yes, the approvals are coming. What we have so far us there on the various product pages. I can add FCC soon. BTL is under way.

Maurice Duteau

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Thanks, the FCC and BTL would be helpful, looking forward to any additional info…

A possible “Work-Around” for the UL listing of the T3-X controllers could be installing them in NEMA1 or Nema12 UL Listed Electrical Panel Enclosures…

  1. http://www.kele.com/Catalog/06%20Enclosures/PDFs/NSTA%20Series%20Catalog%20Page.pdf
  2. http://www.kele.com/Catalog/06%20Enclosures/PDFs/RP%20Series%20Catalog%20Page.pdf

But here it depends on written approvals from the Institutions and Architects / Mechanical Engineers…

A preliminary topology diagram…

Technically speaking, the controlllers are classified as “low voltage” devices. Anything under 40v, as long as it’s powered by a UL listed class 2 (which means it’s energy limiting or fused) transformer then the electrical inspector will not give you a hard time.

I realize owners and the specifying engineers ask for UL though so we are in the middle of getting UL on a number of items.

Maurice Duteau

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I’m currently experiencing ONLY Modbus Communications, NO BACnet communications over the SUB RS485 Bus of the T3-Nano. The MAIN RS485 Bus has both BACnet and Modbus (Master and Slave) communication with two Tstat8’s and sometimes a T3-8o. Firmware as shipped was 49.6, updated to 50.0 and Bacnet MSTP Mode of SUB RS485 bus did not operate for either version. T3000 is the current Nov. 15, 2019 revision.

Is this the normal operation of the T3-Nano?

There are no “found” scanned (BACnet) devices nor Comm LED’s on the T3-Nano’s SUB Bus in “BACnet Mode”, but Modbus configured devices and the Comm LED’s are active in “Modbus Master” Mode. It operates as if it was set to “UNUSED” when configured for BACnet MSTP.

The Main RS485 Bus of the T3-Nano works with both BACnet and Modbus Devices. The devices are discovered when scanned by the T3000 software and YABE…

The SUB RS485 Bus of the T3-Nano works for and is only active for Modbus (Master).

You are probably right, for some reason I remember that Chelsea has disabled Bacnet MSTP on the ‘Sub’ subnet. I will ask her to enable Bacnet MSTP on both main and subnet.

[Update] Chelsea just confirms that Bacnet works on BOTH RS485 ports, the ‘main’ and the ‘sub’ networks both support bacnet and Modbus. Make sure you have updated to the latest firmware to take advantage of this.

Thanks for the reply.

While attempting to activate both Modbus (Sub @ 19.2K with HUM-N-LCD) and BACnet (Main @76.8K with Tstat8’s) on the T3-Nano both became unstable. The T3-Nano operation was stable with only the Main bus configured as either Modbus or Bacnet, which is fine for my current application using one (1) BACnet MS/TP bus for multiple BACnet Devices.

My intent was to use the T3-Nano as a BACnet IP to BACnet MS/TP Router, which is currently working in early trials but the T3-Nano does not have BACnet Explorer (YABE) discovery of several typical “BACnet Router” Variables (BASrouter);

yes… Device Name: (BacnetRouter)
no… Device Instance: ()
no… Device Description: ()

no… Ethernet Network: (0 - 65534)

no… BACnet/IP UDP Port 1: (BACO / 47808)
no… BACnet/IP Network 1: (0 - 65534)
no… IP Address: ( -
no… IP Subnet: ( -
no… IP Gateway: ( -

no… MS/TP MAC: (0 - 127)
no… MS/TP Netwok: (1 - 65534)
yes… Max Masters: (1 - 127)
no… Mac info Frames: (1 - 100)
no… MS/TP Baudrate: (9.6K - 115.2K)
no… MS/TP Tolerance: (?)

MAC Address:

Using YABE most of these variables (IP and or MS/TP) are accessible from the Tstat8 and the T3-8o. These variables may be accessible from the T3-XB’s, but I had not been able to establish uninterrupted communications to my T3-BB which has been returned for replacement. Without access the “Network Variables” cannot currently be easily mapped to another front-end software e.g. Mango Automation. Are there any plans to make these variable accessible from a BACnet Explorer? Thanks.

BTW, the T3-8o works “seamlessly” using the Mango Automation Software (HTML BMS Front-end), All the TStat8 points are discovered / visible / can be mapped , but the operation of the T3-Nano has been intermittent (only connected approx. 85% of the time).

We can add more bacnet objects, no trouble to do that. We’re working on (re) activating bacnet for the subnet which you already pointed out.

Thanks for your work with the testing, we want to make this unit useful in more general applications such as this router task.

The T3-8o “Visible” BACnet Objects are OBJECT_ANALOG_VALUE:1 thru OBJECT_ANALOG_VALUE:17;
5. IP_ADR_1
6. IP_ADR_2
7. IP_ADR_3
8. IP_ADR_4
10. SUBMSK_2
11. SUBMSK_3
12. SUBMSK_4

Most of these OBJECT VARIABLES are not currently visible in YABA for the T3-XB (LB’s) which are currently installed and in trial…

From my current testing the preliminary Topology Diagram is;

Topology: Any particular reason you have that 4th floor thermostat bus on Modbus while the rest of the system is on Bacnet?
Bacnet Objects: We’ll add some AVs for the T3 series controllers like we have done for the other products: Communications settings, IP address and so on.

No reason at all for the Modbus (for my current application), it is just to show that a “Modbus RTU” could be implemented on the MAIN RS485 Bus.

@maurice ,

Is there any detailed info for EOL Terminations or Biasing of the Temco RS485 MS/TP Bus for Tstat8’s and other Temco BACnet devices? Thanks.

T3000 Software and YABE are able to see (discover) multiple BACnet devices on the T3-Nano MAIN bus, but other BACnet Explorers (Mango and Wacnet) can only see the 1st Device (a Tstat8). I’ve had similar situations when a Lon Bus did not have an installed termination resistor, one such Lon Bus has been extended to over 2 Km using an existing Telecom Line (Cable) with a single properly located EOL Resistor and no repeaters and has been operational with 100% communications for over a decade.

BTW a 120 Ohm 1/4 watt 1% resistor (EOL) makes the other BACnet devices visible with the other BACnet Explorers, but interrupts communications in the T3000 software. And the T3-8o has been the only device that has been communicating 100% across all the tested software.

Hard to say what might be going but here’s a couple of suggestions on how to debug deeper.

I wrote an article on how to capture MSTP traffic with Wireshark which will get you a log of the traffic which we can dig through

For termination issues you can have a look at the RS485 signal traces and get some clues. Here’s a little pocket scope which can show the A - B trace which is what you want for RS485 signals.

Standing by to help more.

Thank for that info.

Looking to get some info on the designed bias and termination resistors, because I only have an RS485 BACnet MS/TP Buss with less than a total 10 feet / 3 meters of 18 Guage twisted shielded wire and a maximum of four (4) BACnet devices, including a T3-Nano or T3-LB with intermittent MS/TP communications.

Four devices on a short network like that, you shouldn’t have to worry about termination at all. If there are communications issues it could well be at the data layer level rather than the RS485 signal level. That means the wireshark capture would be useful as a first step.

@PeteR what did you end up doing with your testing? I’m curious

There’s a lot of testing going on here all the time: communications, sensor accuracy, power consumption, high voltage tests. Which test are you referring to?

@bill ,

I’m still testing the Temco devices. The Tstat8’s RS485 Bacnet MS/TP bus is connected to a T3-TB which has been much more reliable than the T3-Nano. They are bench mounted on Din Rail with several Temperature, Humidity, Light Sensors and ice cube type IDEC Relays (for DO and DI testing simulating Motor Starters and or VFD’s, with status feedbacks). The Tstat8’s will be installed on a couple OpenAG MVPs as the “Brain” with the Raspberry Pi 3+ as a monitoring computer.

The BACnet IP bus devices, T3-TB and T3-8o have reliable communications with T3000 software, YABA and the other BACnet “Explorer” software programs.

T3-TB and T3-8o have reliable BACnet MS/TP communications with T3000 Software and YABA but do not have reliable communications over their respective BACnet MS/TP bus with the other BACnet “Explorer” Software (the Tstat8 BACnet Objects are not readily discovered with the T3-TB or T3-Nano as Routers / Gateways).

Also the Tstat8’s RS485 Bus always has to be installed as 3 wire (terminated ground) as shown in the Temco documentation for the most reliable communications; https://temcocontrols.com/shop/tstat8-bacnet-thermostat/