SOLVED Problem with second monitor and imac

[imacchris]

I have a Samsung S24B300 monitor connected to my 2017 imac (OS Monterey 12.4) with an HDMI cable.

About 8 times out of 10, when I start my mac, both monitors (on imac and S24B300) come up with not problems. The other times, the S24B300 does not light up. If I unplug the S24B300 and replug it, it lights up and everything is ok. (Just turning the S24B300 off and on again does not work.)
If the mac goes into sleep mode, when it wakes up, about half the time the S24B300 remains dark and I have to plug/unplug it once or twice to get it going again.
I have used several HDMI cables but still have the problem.
Can anyone know what could bve the problem and how I might solve it?
Thnks

 

[Cory Cooper]

Hello and welcome.

I have seen this issue on multiple Macs running multiple different non-Apple displays. I believe it has to do with the HDMI handshake. I don't know of the actual cause or any permanent fixes, as I experience the same issue intermittently.

You could try an SMC and/or NVRAM reset.

Wish I had a better answer.

C

 

[lajay007]

We have had this problem on a bunch of MacMinis. The SMC test and NVRAM don't help. The only thing that's helped us is to reseat the HDMI at the computer. Yes, we have to do it every boot up.

 

[Howard Brazee]

I have an M1 iMac. It worked with multiple HDMI monitors until MacOS upgraded (maybe to 12.3), with Universal Control. After the "genius" tried a lot of stuff, including reinstalling, they said my iMac didn't support multiple monitors.

But I use multiple HDMI monitors today. The trick is not going through Apple. OWC has a couple of devices that use "DisplayLink" software that support multiple monitors. I bought https://eshop.macsales.com/item/OWC/CADPDL2HDMI/ which has 2 HDMI plugs, but I read that now it has an external bus that has HDMI plugs and other plugs (which I would prefer). OWC support was good, I asked them for what I needed and they told me what to buy (and where to download DisplayLink software).

 

[wab123]

I have a Samsung S24B300 monitor connected to my 2017 imac (OS Monterey 12.4) with an HDMI cable.

About 8 times out of 10, when I start my mac, both monitors (on imac and S24B300) come up with not problems. The other times, the S24B300 does not light up. If I unplug the S24B300 and replug it, it lights up and everything is ok. (Just turning the S24B300 off and on again does not work.)
If the mac goes into sleep mode, when it wakes up, about half the time the S24B300 remains dark and I have to plug/unplug it once or twice to get it going again.
I have used several HDMI cables but still have the problem.
Can anyone know what could bve the problem and how I might solve it?
Thnks

 

[wab123]

imacchris, I can't help you however, I upgraded to an M1 MacMini, now on 12.4. I run a 27" HP monitor, no problem or, problems. I connected a second monitor, a21" Samsung S24B300, plugged it into the back of the MM and it works fine and they light up together from 'sleep'.
The M1 however is a cranky damn thing, it's internals change with every update, why can't Apple leave well alone ?

 

[imacchris]

Thanks for you replies. I was beginning to think that my problem was unique.
I have reset NVRAM several times before but that didn't seem to make any difference. Resetting SMC improves things a bit but the problem seems to be related to OS updates. The monitor worked without a hitch back in early OS 11 (I think) and since then the problem gets worse with every update. Maybe I have to reset SMC after every update to minimise the problem.