Led matrix communication design - Page 1
Forum section: electronics / projects

author: BorisRap
author_rank: Contributor
timestamp: September 07, 2023, 07:56:08 am
content: Good day,I'm trying to design a wall mounted display.It consist of modules 30cm x 30cm the modules are put side by side in a matrix of 27x18 modules (486 modules), the display would be in total around 8x5m (26 x 16ft).Each module has a controller and a RGB led matrix. It consists of a PCB with the components, a frame and a diffusing layer. All modules are identical.The question is about the communication interface I should use to address every module.Here are some parameters I've set:I want a minimum refresh rate of 30HzThe modules are always listening to the com bus and they only talk when asked.I've set a control packet size of 10 BytesI need to be able to broadcast to control all modules (or groups) at the same timeI need to be able to control every module individually at min 30HzThat means, if I want to send 10B to 486 modules 30 times per second I need minimum 1.2 Mbit/secondI tough I make every module identical and I pass the communication and power delivery lines from one board to another like so:I have not much knowledge in digital signals and transmission lines but I'm aware of the problems of reflections that this system poses. Specially at these kind of distances.I am flexible to try other systems or techniques as long as it doesn't involve wiring each module individually.I could put more data lines and make the controller talk simultaneously in all data buses to many modules, that would reduce the bit rate in each bus.I can use differential lines and other things like that but the solution must be rather cheap. Some isolation IC's are crazy expensiveDo you have some ideas on how I can approach this problem?Thank you for your insight!

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 07, 2023, 08:15:08 am
content: Definitely differential - RS485, split into multiple buses, no need for isolation if everything runs off the same supply. 2-4Mbaud.However probably a lot cheaper to use off-the-shelf video wall or signage modules.

author: BorisRap
author_rank: Contributor
timestamp: September 07, 2023, 09:44:49 am
content: Quote from: mikeselectricstuff on September 07, 2023, 08:15:08 amThanks for your answer, I though about using RS485, the thing is RS485 doesn't work with this matrix configuration. I have to connect the panels in a way that they're in a line one after another. (which is not terribly bad).The total distance if I put all panels in a line is around 160m (530ft) which is not really bad for RS485, but the cables are actually traces in the boards, and I cannot twist them.Plus there is a connector between every 2 boards (adding impedance to the line)Do you think it will still working around 1-2Mbs?Thanks again

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 07, 2023, 09:51:47 am
content: Quote from: BorisRap on September 07, 2023, 09:44:49 amFor 160m, 2mbaud is doable but using decent twisted pair cable not cat5/6 - line resistance is typically the first limit you hit so you want to be using 24/22AWG cores.Is it not possible to use a common bus and assign IDs to the panels instead of using a chain ? - that way a failure will only affect one panel, not everything downstream.If you do it as a chain, then the links will be short as you'll be re-generating the data on each hop, so length/baudrate shouldn't be an issue.

author: BorisRap
author_rank: Contributor
timestamp: September 07, 2023, 10:38:57 am
content: Quote from: mikeselectricstuff on September 07, 2023, 09:51:47 amYes, that was the idea, use a common bus and branch every device.What is not negotiable is to use cable. I cannot wire every board and split the bus for every module. The bus must be the copper traces of the pcbs

author: BorisRap
author_rank: Contributor
timestamp: September 07, 2023, 10:49:56 am
content: Quote from: mikeselectricstuff on September 07, 2023, 08:15:08 amWould be possible to use SPI and add a buffer to each board? I know I'm adding propagation delay at every buffer, but If I route it correctly It might be small enough to be able to manage it.In that case, the SPI line from buffer to buffer would be at max 30cm.

author: coromonadalix
author_rank: Super Contributor
timestamp: September 07, 2023, 02:12:57 pm
content: mmmmm what are your led modules / tiles P3 P4 P7.68 P10or something else ?you have very fast controller for already made panels, and they can output high count pixels in the 4thousand or more ? live videos etc ...

author: mariush
author_rank: Super Contributor
timestamp: September 07, 2023, 02:46:25 pm
content: Another option could be plain ethernet...You could give each module a unique ID (2 bytes, for a maximum of 128 x 128 tiles or 254x254 , use 255,255 for broadcast - as in packet for all the modules)There's 24 port 10/100 mbps switches for 10-25$ each on eBay and 48 port switches are as low as 30-35$, with 11 48 port switches or 22-23 24 port switches you'd give each panel up to 100 mbps.Considering the amount of power the leds would consume ... the 100w or so 20-30 switches would consume is nothing.In 10 mbps mode, you only need 4 of the 8 wires in an ethernet cable, so you could easily make your own ethernet cables that split into two ethernet ports to save money on cable or you could use the extra 3 wires as passive power over ethernet to send power to each panel (ex 24v 2-4A per panel or whatever)Each module could have a ethernet to SPI module that connects automatically to a stream (udp, tcp) and ignores any packet that's not for itself and once every 33 ms you send a packet that triggers all panels to refresh.

author: tychob
author_rank: Contributor
timestamp: September 07, 2023, 03:02:54 pm
content: Quote from: BorisRap on September 07, 2023, 10:38:57 amHow are you handling power distribution without cable? How much current do the boards draw?Because at an out of the air conservative guess of 20mA, that would result in the supply needing to draw 10A if it comes from one place which would be non-trivial on a PCB. If its 2-4A at 24V as marlush suggested, then your power grid needs to deal with 23-46kW. Thats the kind of power levels that require thought to distribute at mains voltage levels, you would need cable and segmented supplies. The only time I've ever worked with a screen like this, it had a dedicated cable from the control rack to the panel, for every section, in order to distribute the power. If we know more about the power deliver network, that might expose some alternative options for a control plane.

author: mariush
author_rank: Super Contributor
timestamp: September 07, 2023, 03:35:21 pm
content: OP never said how many leds the tile has, hence why I was just saying an average number.For example, here's a 144 led (12 x 12) tile using 5050 leds at a 25mm pitch - works out to 12 x 25 = 300mm or 30 cm by 30 cm tile :https://www.adamhall.com/shop/bn-en/archiv/2153/kling-tile-144As you can see the power is advertised as 65w - that's 3A at 24v ... hence why I said 24v 2-4A of course you could do brightness tricks to dim the whole panel when lots of white is shown or something like that, to reduce the power consumption.Another one with just 64 pixels ... advertises 72 watts :https://www.av-iq.com/avcat/ctl1642/index.cfm?manufacturer=traxon-technologies&product=64pxl-tile-matrix-rgbCan't pass through pcbs thousands of watts if you're gonna have 28 tile wide screens. You'll need some power distribution through... and from a power conversion point of view it makes more sense to have power supplies shared by multiple modules instead of having a power supply in each module.For example, you could have a 1000-1200w ATX power supply (12v 120A) power 18 tiles (65w per tile) by using customized EPS or pci-e connectors - a 1000w psu will have at least 5-6 headers with 3 pairs of 12v+ground , and you could also use the peripheral headers if needed.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 07, 2023, 03:45:39 pm
content: If you use daisy-chain, the signal will be regenerated by each device for the next one, thus allowing high speed signals and virtually unlimited devices.That's how addressable RGB leds do (WS2812, etc ).In fact, better that you use them for you led wall, they are cheap and very popular, so you'll find lots of resources, libraries and projects for them.How many RGB leds do each module have?10bytes is nothing, if you have to update the entire module array it'll be a lot more, you have to think in the worst situation, which is updating the entire wall at 30fps.It'll be a lot of pixels!

author: spostma
author_rank: Regular Contributor
timestamp: September 07, 2023, 04:14:18 pm
content: Did you consider using a controller and LED tiles that use the HUB75E interface?IMHO that is the easiest way to make a video wall.The controller has a daisy-chainable gigabit ethernet interface,and accompanying Windows software can be downloaded from the colorlight-led site as well:http://www.colorlight-led.com/download/colorlight-5a-75e-led-card-specification.htmlhttps://nl.aliexpress.com/item/1005005491817855.htmlhttps://nl.aliexpress.com/item/1005005207613535.htmlThere also exist HUB75E interface using raspberryPi and ESP32 using their DMA,but these are for smaller number of tiles.

author: tychob
author_rank: Contributor
timestamp: September 07, 2023, 04:39:46 pm
content: Quote from: mariush on September 07, 2023, 03:35:21 pmExactly. And if you need a separate power cable to every 18 tiles and piggy back off that to add a control plane line? Then that's only 43 Kb/s, and you can get away with a lot at that rate, along with easier routing if it only needs to go to 18 tiles.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 08, 2023, 12:18:37 am
content: Quote from: BorisRap on September 07, 2023, 07:56:08 amYou should also look at no wired junction for coms at all.With so many panels, the best designs I've seen did power BUS BAR connections and had no physical wires panel to panel.Instead, they used optical or inductive couplers, making the link much more reliable.Like yours, these panels are usually quite large physically, giving many choices for non-physical coupling.Parts like LT1328 can manage 4Mbps optical links

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 08, 2023, 02:28:23 am
content: Quote from: BorisRap on September 07, 2023, 10:49:56 amSPI has no common mode tolerance, and needs many contacts, so it is less ideal here.