onsdag den 10. september 2014

A ROM substitute / development tool

So, I have run into the problem of needed programmed devices for my computer projects. A CPU cannot (mostly) start proper operation without some lines of code. Thus raising the problem: how to test your system without a ROM? Well, i was reading up on the 74154 IC. It's a 4 to 16 line decoder. So i got the idea, of making a diode prom with it, using four address lines, eight data lines, and dip switches with diodes. The solution looks like this:

An address is entered on the An to An+3 (you can use whichever four addresses the system reads first, it varies with processor) and then, the two chip select and output enable (they're not called that on the 154, i've made the pinout compatible with the 2716), connected as well as the data lines, and then on the 16 x 8 dip switches, you can enter the programs or routines you want. 16 bytes seems infinitely small, but plenty of room for some test, and a HLT or so. a counter maybe. The 74ls240 is just an 8 bit inverting tru state buffer. You might need to connect the /G lines which disables output to /READ. 

Note: This is kinda WIP, i haven't tested or even built this yet, so I'm just throwing it out here for the interested.

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