Connecting two Displays

The Dot Matrix Display (DMD) is a 32x16 array of high-brightness LEDs for visually striking effects. [Product Page]
hersh
Posts: 7
Joined: Wed Jul 04, 2018 11:58 pm

Connecting two Displays

Post by hersh » Thu Jul 05, 2018 12:03 am

I have two 32x16 DMD panels and have them connected pin for pin. The example Marque Arduino Sketch has been changed to 2 screens wide but the scrolling shows nothing on the second display. Any help will be appreciated.

Brissieboy
Posts: 133
Joined: Fri Sep 20, 2013 7:25 am

Re: Connecting two Displays

Post by Brissieboy » Thu Jul 05, 2018 4:27 am

Not sure what you mean by "pin for pin", but DMDs need to be connected in a daisy chain with the Arduino connected to the last DMD using the one-to-one ribbon cables supplied with them.
Have a look at this.
If you do have them connected correctly without success, try the demo using just a single DMD and make sure both are working. And make sure you have them powered by a suitable 5V supply - you need MINIMUM of 3 Amp supply for 2 DMDs. Do not run these from your USB without external power to the DMDs.

hersh
Posts: 7
Joined: Wed Jul 04, 2018 11:58 pm

Re: Connecting two Displays

Post by hersh » Thu Jul 05, 2018 4:51 am

Thanks for the prompt reply. Yes using the equivalent of a one-to-one ribbon cable. Both displays are working. I have a third which isn't. I run the brightness at a minimum so not consuming too much power. One DMD is fine being powered from the Arduino. Can even turn on all LEDs at low brightness. I will also try your power suggestion later tonight and let you know how I get on.

hersh
Posts: 7
Joined: Wed Jul 04, 2018 11:58 pm

Re: Connecting two Displays

Post by hersh » Thu Jul 05, 2018 7:08 am

You were right. Powering the second display did the trick. My sample code uses 430 mA for each display so it makes sense that the second display didn't come up. I'm surprised the first one did. Once again, thanks for your help.

Brissieboy
Posts: 133
Joined: Fri Sep 20, 2013 7:25 am

Re: Connecting two Displays

Post by Brissieboy » Thu Jul 05, 2018 10:57 am

If you check out the circuitry and interconnection between the Arduino and the DMD you will notice that there is NO 5V power rail connection. If you do not supply power to the DMD then it can only be sourced via the asserted interconnections which then feed power to the 5V rail of the DMD via the protection provided on the 74HC245 bus transceiver. This has 78connected lines, not all of which are at logic high at any one time, so the entire DMD load is fed in this way by a limited number of input lines. The absolute maximum current rating for these lines on the 74HC245 is 20mA.
All this power is supplied by the Arduino outputs which also have a max rating of 20mA, and there are only 6 of these used for connection to the DMD (2 lines each drive a pair of the 74HC245 inputs).
At the very least, you are placing both your Arduino and DMD at significant risk by attempting to drive them solely from the USB port.
The main reason the second DMD will not operate is due to the voltage losses along the way. The Arduino output high will be less than 4.5V (probably a lot less because of the load as it is max 4.5V at 20mA), drop at least another 0.7v at the 74HC245 leaving something like 3.5V at the Vcc rail of the DMD - and that is assuming the USB voltage is at 5.0V with this added load. A second DMD connection will be driven by the output of the 74HC245 which will be something less that the DMD rail (at least 0.5V less), add additional 74HC245 loss on the second DMD as well as adding additional load on the other DMD and Arduino further dropping the available voltage. It's anybody's guess what the supply rail on the additional DMD will be, but it will be way down.
Get your multimeter out and check a few voltages.
I know the practice of simply using the USB to power an Arduino plus DMD has been promoted in a lot of places, including on this forum, but is just WRONG!!
ALWAYS POWER YOUR DMDs.

hersh
Posts: 7
Joined: Wed Jul 04, 2018 11:58 pm

Re: Connecting two Displays

Post by hersh » Thu Jul 05, 2018 12:01 pm

Thanks for the explanation and tip. Now I need sunglasses to read the display. Is there a way of dimming the panel? I know DMD2 has one but I've discovered that using an IR sensor and DMD2 results in inconsistent IR readings - so I need to use the original library. But how to dim the display?

hersh
Posts: 7
Joined: Wed Jul 04, 2018 11:58 pm

Re: Connecting two Displays

Post by hersh » Thu Jul 05, 2018 12:04 pm

Can I run it off 3V instead of 5?

User avatar
stryker
Posts: 268
Joined: Sat Jan 14, 2012 2:44 pm

Re: Connecting two Displays

Post by stryker » Thu Jul 05, 2018 12:50 pm

No, 5V for these ones

Brissieboy
Posts: 133
Joined: Fri Sep 20, 2013 7:25 am

Re: Connecting two Displays

Post by Brissieboy » Thu Jul 05, 2018 11:16 pm

Using 3V will still load your Arduino and 74HC245 on the DMD. I strongly suggest NOT going that way.
DMD2 brightness control should work. If not, have a look at this link for control of the brightness (but for DMD not DMD2)?
There should be no issues controlling brightness using an LDR, but an IR sensor would not be appropriate for this.

hersh
Posts: 7
Joined: Wed Jul 04, 2018 11:58 pm

Re: Connecting two Displays

Post by hersh » Thu Jul 05, 2018 11:48 pm

Thanks for the link. Will try it over the weekend. The IR remote is for updating the scoreboard I'm building but as reported on this Forum there seems to be an issue when using DMD2. Your suggested solution looks much easier.

Post Reply