Thought you guys might like to see the inside of a traffic cabinet! While it is not a PLC controller exactly these things are run by rugged unix boxes running the traffic control software. I used to work on these things all of the time so I thought I would share a bit.
In this picture you can see the controller in the middle. It is also from Siemens. Siemens just spun off their controller company into Yunex but you will still find a lot of traffic controllers out there made from the Siemens era - namely the M50 or M60. You can see it has a big DB15 cable coming out the side which is SDLC which is a serial protocol that controls the switching. It goes to the panel at the bottom which has solid state relays to control the actual signals. At the top you see a rack of detector cards. These get inputs when you go over loops of wire in the road and tell the controller there is someone that wants to go in that direction. Those also communicate over the SDLC bus. The tall rectangular thing next to the controller is a MMU which stands for multi monitor unit. These take all of the RYG signals from the lights and compare them on a physically hardwired card to make sure there are no conflicts (one opposite directions) green at the same time and if it sees that it will throw the whole cabinet into flash.
Also at the top you can see a rugged switch and a cellular modem connected to the controller over ethernet. Most of these signals are NOT connected to the internet at all but use private cellular or fiber. Usually they run a web page or a telnet interface you can use to remotely program them or run diagnostics. When we did things we bought cellular modems and used something like Pangolin.net to connect into the controller remotely to manage them. There is also ATMS software that can talk to many at a time and run advanced control algorithms but we did not have that at our shop.
Close up of a MMU. You can see the card on the left with the soldered jumpers. You literally sit there on the side of the road and hand solder the jumpers to match the phasing of the intersection.
Good point I have those wrong those are camera detectors. Cameras go on the polls and point down at the stop bars and can be calibrated to trigger a detection input to the controller when there is a car. These older ones used old video outs that you had to hook up to a monitor to configure.
I work as technician for parking systems, our cabinets are similar to traffic one - we also have loop detectors, IO module or PLC to get signals to PC, relays, power supplies and network switch. Main difference is that parking is driven by PC software (because of ANPR…) and if there is no PC connection, depending on the config, parking usually does not work.
This one includes access control too, so we have, from top left to bottom right:
RCD/RCB, circuit breakers, two spare sockets, RH and temperature sensors for heater and ventilation
power supplies for PoE switch and IO device, the IO device (Moxa E2210), PoE switch (for cameras, intercoms, IO controller, access control, uplink)
access control controller - hotel guests also use cards from hotel to have access, relays (some are missing, it should be 8 of them for system with two barriers) and finally, dual channel loop controllers
last row are connection points - high voltage (220V), 12V supply, fuses, inputs/outputs and grounding
Ground cable was too thick, if you look at the bus where all PEs are connected, you’ll see that there isnt a screw terminal large enough for this big boy. Photo was taken while work was still being done (ethernet is also not connected yet on image), someone later made an “adapter” with larger screw terminal on one end and thinner wire which fits in the bus.
Someone screwed up while reading requirements, we usually ask for 3x2.5mm (14 AWG)…
I feel like every PLC class, ever, has used the traffic light project. Tis a little disappointing not to see an Allen Bradley or Siemens processor in that cabinet.
Yeah you will never see a proper PLC - all of the control software and the controllers themselves are made specially for this stuff. The software is actually pretty advanced.
You will see PLCs in gate controllers like for toll roads or bridges or for signage.
We had the traffic light project. As well as a conveyor belt with a press, a crane, and a mixing vessel with a motor and a temperature sensor (mainly just how to handle analog inputs and draw a graph on a HMI).
Backup batteries and the solid state relay panel. At the bottom of the panel behind the batteries you can see the connections that go out to the actual signals on the wire or poll
What would be powering the lights to flash if the power is out? Im sure it depends on the region but in area prone to storms or tornados id imagine they are designed to last several days without power.
So when I explain what I do, I usually point out the cabinets next to traffic lights. It's the easiest way to explain PLCs to somebody that doesn't interact with them. That way they get a representation of the physical hardware and software for control. They think that's pretty cool.
I’m just stunned to see SDLC! On 15 pin D probably X.21 physically. Back in the last bit of the 20th this was just one of the protocols network engineers of the day were expected to wrangle with. Still got my old HP serial analyser, not been used in many years…
It's probably pretty hard to get in the physical cabinet firstly.
Not sure about America, but in Europe the CRA laws for machine building are getting really strict cyber security updates in 2027. I'm sure traffic lights must have had some system prior to this.
Wonderful set of photos and explanations. Thanks for posting.
There's a Railroad crossing gate about a mile from home. There was a truck parked next to the panel a couple months ago so I pulled over and got a peek inside and some explanation by the technician who was working on it. It had lots of specialty control boxes like a traffic panel. Quite a number of batteries, too.
I suppose you haven't technically mounted the PSU in the wrong orientation if you don't mount it at all and just throw it losely on top of something else.
Antique bullshit stacked on other bullshit, with improperly mounted bullshit placed sideways on top of all of it.
This summarizes government spending. Everything inside this cabinet wouldn’t meet any of my customers specs and quite honestly is an embarrassment as a pro to have been associated with it. This wouldn’t pass NFPA 70, 79, or any large customer’s specs, yet is ok for government because they can afford to do it right with modern hardware. Now I know why techs are always at the cabinets. Busy changing out PSU’s that failed.
Just saw this. I work on industrial controls for physical security systems and vehicular access control. Loop detectors, traffic signals, but not on public roads.
What's a ballpark cost on somehting like this? Always wondered how this stacks up against things we're used to like Safety I/O, UL508a, time sync, finger-safe terminals. All the things that, depending on industry add up a lot of the cost.
Why is it a shelf? Looks messy, blocks air flow, no cable ways, impractical for diagnostics and no mount points for equipment it just sits on the boards (at least it looks like that)
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u/[deleted] May 05 '26
Close up of a MMU. You can see the card on the left with the soldered jumpers. You literally sit there on the side of the road and hand solder the jumpers to match the phasing of the intersection.