6546 or 6845

[Gary C]

My understanding is that any 6845 can be used in the PET as the extra registers aren't used in the PET ?

I have a 8032SK on the bench with a failed 6545 and I have replaced it with my spare 6545 but would rather use the cheaper 6845 for this machine and keep the 6545 as a spare for my SK.

I have just received a MC6845P and it doesn't work but before I return it, I just wanted to check ?

 

[This.is.Matt]

Didn't know that the PET had anything to do with Acorn ...

 

[Radix]

I remember getting 10 6845 from a seller on Ali Express and all 10 were faulty and non-working...

Robin

 

[Eudimorphodon]

My understanding is that any 6845 can be used in the PET as the extra registers aren't used in the PET ?

There seem to be a lot of threads that say 6845s work fine in PETs, but it does seem like there are anecdotal references to at least some Motorola 6845s not working? It doesn't seem like anyone's ever done a detailed taxonomy on which ones might be a problem. This Acorn fan has put together a list of what he's identified as the actual feature differences between the different 6845 variants out there, (and here's another from 6502.org) but I don't *think* any of these would affect a PET?

 

[Gary C]

I have a pile of them now, MC6845P (ie not the -1) and none of them work in the PET or the BBC (well, they work in all modes except mode 7)

Looks like you need the Vertical Sync Width Register.

 

[Pernod]

I have a pile of them now, MC6845P (ie not the -1) and none of them work in the PET or the BBC (well, they work in all modes except mode 7)

Looks like you need the Vertical Sync Width Register.

The BBC really needs the HD6845 variant for the way in which the interlaced MODE 7 is configured.

 

[Gary C]

I will try a BBC compliant unit in the PET as I suspect it will work as I am sure I tried it before but wasn't aware of all the differences then.

All I know at present is that a MC6845 that doesn't work with Mode 7, doesn't work in a working PET.