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Using FLOW with a Luxcom HDLC converter (SDLC) - adaptive traffic control data communication
Using FLOW with a Luxcom HDLC converter (SDLC) - adaptive traffic control data communication

Learn how to connect FLOW to the Luxcom EM-HDLC converter, which can communicate with traffic controllers over SDLC.

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Written by DataFromSky
Updated over a week ago

Did you know that FLOW can help control intersection traffic lights? Each such intersection should be controlled by a traffic controller. If this controller supports the NEMA TS-2 standard, it communicates over the SDLC bus with terminal devices that help gather information or actuate parts of the intersection. The Luxcom EM-HDLC converter can serve as a detector device that can receive vehicle presence information from FLOW and send it to the traffic controller, which can use it to optimize its traffic light cycle length for the best possible vehicle throughput.

Traffic Cabinet Basics

To best understand the role of a FLOW device and the converter in intersection control, let's start with a brief overview of how a traffic cabinet works. If you know or don't care about the theory, you can skip this section without much trouble.

The NEMA TS-2 traffic controller communicates over the SDLC bus with terminal devices.

Example of a real traffic cabinet:

The cabinet parts include:

  • Control Unit (CU) - the traffic controller

  • Malfunction Management Unit (MMU) - takes control of the cabinet in case of CU malfunction

  • Terminals and Facilities Bus Interface Units (TF BIU) - they forward commands from the CU towards actuators like load switches, flash transfer relays and flashers

  • Detector Rack Bus Interface Units (DR BIU) - collect and forward information from detectors (induction loops, smart cameras) towards the CU

These parts are connected by a single full-duplex Synchronous Data Link Control (SDLC) bus, called Port 1 in the NEMA TS-2. The bus is full-duplex - the CU transmits on one channel and all other devices transmit on the other channel. Therefore there's a need for synchronization, so the other devices don't transmit at the same time. The CU addresses specific devices with requests and the devices then have a timeout of 0.5 milliseconds to compose and start transmitting a response.

The protocol's complexity and expected reaction times require specialized devices that serve as an interface between the SDLC bus and peripherals. These devices are called Bus Interface Units. A typical BIU is connected to the SDLC bus on one side and to a rack backplane on the other. In the case of DR BIU (which is the main focus of this project), the backplane serves as another bus for detector cards, which serve as interfaces for one on more detectors are connected to them.

Below is an example of a detector rack. The BIU is on the left. The other slots are detector cards. The backplane is behind the BIU and cards. All of them are connected to the backplane.

An example of a detector rack. The BIU is on the left. The other slots are detector cards. The backplane is behind the BIU and cards. All of them are connected to the backplane.

The Luxcom HDLC Converter

As mentioned earlier, the a FLOW device by itself isn't able to meet the requirements to communicate on the SDLC bus - mainly because the response timeout is too short. That's why there's a need for a SDLC converter. The Luxcom EM-HDLC converter has an SDLC connector and functions as a DR BIU on the CU side - this means it automatically responds to Call data and Diagnostic requests from the CU. A FLOW device can connect to its Ethernet port. By sending UDP packets in a specific format, it can change the content of the converter's responses on the SDLC bus.

The NEMA TS-2 standard defines up to 4 possible DR BIUs, each BIU with 16 detector channels, therefore up to 64 detectors total. In the original design, the detectors are supposed to be induction loops hidden under the road. FLOW and the converter have to somewhat emulate this behavior. This means that detector outputs are limited to simple "true/false" values, meaning "vehicle presence detected" and "vehicle presence not detected" respectively. However, as you'll see later, FLOW gives you great freedom in deciding how the true/false value is determined.

You can find more info on the converter in the brochure. Some more details are also in the user manual, which you can obtain on the Luxcom website or ask them for it over email, if you don't have the password. Getting the firmware user manual is also recommended, but this article will try to explain the setup without you needing these manuals.


For demonstration purposes, this guide presumes that there is at least one working analytic in your FLOW setup, that there is a zone filter in this analytic, and that there is a Value widget attached to this zone.

Now open your web browser and type the converter's IP address into the address bar. You should be greeted by its web interface. Next:

The default username is admin and default password is luxcom.

Now open FLOW Insights, connect to your FLOW device, select Interfaces and Add an interface.

Fill an arbitrary interface name, fill the IP address of the converter and the same port numbers that you configured it with. Leave DR BIU ID at 1 for now and select Save settings.

You should now see that the Status switched to Online. This means that FLOW is able to send commands to the converter and receive valid responses.

Let's set up some detectors. Select the Edit button on the right.

Now select the Edit button of one of the channels.

Here you can enter a Javascript expression that needs to evaluate to a boolean value (true/false).

In our example, we selected one of the widgets from the bottom, filled in the objectCount() attribute, and put in the condition that this value must be greater than 0. By this we effectively say "number of traffic objects in the zone must be greater than 0".

Now you can see that the channel's value changes in real time based on what's happening in the scene in the zone the widget belongs to. If there's a traffic object in the zone, the Call request value becomes true.

This effectively sends a true value on DR BIU 1's channel #1 to the traffic controller, notifying it that there's a vehicle in the detector's zone and it requests a green light. How the traffic controller responds to this call and how it knows what intersection phase the detector belongs to depends on its configuration, which is beyond the scope of this article.


Before FLOW version 1.15.5, Luxcom's firmware manual contained errors. This sometimes results in FLOW not being able to communicate with the converter and stating the interface's status as Offline. If you're using FLOW with version lower than 1.15.5 and you're having trouble with the interface, try deleting and adding the SDLC interface in FLOW Insights multiple times. With each addition, FLOW attempts to connect to the converter again.

Luxcom's manuals and converter firmware have since been updated and the changes are reflected in FLOW v1.15.5 and forward. For optimal stability of this feature, we recommend updating FLOW at least to this version.

If you need further clarification or help, click the button on the bottom right to chat with us or contact us here. We're happy to help!

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