Why Are There Extra Pins on ROM Sockets for Certain Boards?

[jajan547]

Just curious if you were to look at say a Color Classic, Color Classic ii, LC475. Lciii/iii+, Classic, or Classic ii/Performa 200 for example why do these machines have the ROM backed out one row over? Was this because Apple considered ROM upgrades or what is the story here, are different chips compatible with the same ROM but also occupying all of the pin rows?

 

[retr01]

That is a 42-pin socket. Apple specified a 42-pin socket and had the 40-pin ROM dropped there. Maybe a simple design or production mistake that was slid? This oddity has been known since around 1990. Note in the second paragraph of the first column here:

 

[retr01]

I wonder what @max1zzz or @Kai Robinson or someone else thinks about this?

 

[jajan547]

I wonder what @max1zzz or @Kai Robinson or someone else thinks about this?

It would be interesting to hear their perspectives after all of the reverse engineering and understanding they have of these boards now.

 

[max1zzz]

The extra pins are for extra address signals to allow for larger ROM's to be used, this was presumably because apple had plans to release upgraded ROM's at some point but never did

 

[Trash80toG4]

That makes sense to me now. Note that the ROM is installed backwards? Would the higher capacity ROM IC key align with the silk screen key as shown? Very strange, is the silk screen keyed at both ends?