Super old Pioneer plasma - signal not supported?

[Scam]

Help! I've moved into my Dad's a couple of weeks ago and I'm just trying to plug some of my stuff temporarily into his TV. It's a really old (circa 2002 I think) Pioneer plasma whcih although I'm sure doesn't do 1080p, I thought it may do 720p I can't find the model number but it's so old it comes with it's own media receiver which is a Pioneer PDP-R04E.

Everything is going into an Onkyo TX-SR578. He had a Wii, 2 DVD players and an AppleTV going in via HDMI so I didn't expect any problems when I took out the DVD players for my FireTV and my Sony bluray player. But I get an error saying 'Signal not supported' on the plasma for both

Any ideas? Both devices only have HDMI out, I'm hoping I'm not stuck here..

 

[Scam]

Thanks. Yeah it's odd. The AppleTV definitely works over HDMI so it must do 720p.

Because we are only here temporarily I was wondering whether to unpack my Panasonic GT50, but I think all this has answered that question for me! Annoyingly his TV is mounted to the wall so I'll have to cover that with a sheet or something whilst mine can sit on the TV cabinet. That'll look nice