I deeply apologise in advance, this is likely not the place to ask this but I do not know the correct place to ask it!.
I have an Atmel AT89C51RD2-UM Microcontroller, from what I can work out these are quite common way of controlling circuits back in the 50's and onwards.
I want to program it via ISP as its installed in a circuit, which I'm looking to combine with my Arduino Mega, I cannot figure out for the life of me how to actually program it, or what I need to do such a thing, and I'm struggling to find the right place to even ask that kind of thing! If you could help me I would greatly appreciate it!
Considering that microcontroller we’re only introduced in the 1970s I doubt that.
On the Arduino boards there is a 6 pin header for connecting an in circuit programmer. You can make one from an ordinary Arduino by putting the ISP code in the IDE into it. You have to disable the reset pin either by cutting a link or putting a large capacitor across it.