Dumping roms

Let's have a look at the most difficult part of the documentation process: the dumping! I usually go through a 3 steps path: 1)Components identification 2)Actually dumping them 3)Checking what I've done

1- Components identification. On a standard PCB we will find one, more or even all of the following components: CPUs, TTLs, ROMs, RAMs, PLDs, Others. I use coloured water-markers (of the kind used to write on CDs) to mark each single component when it is correctly identified: green is for CPUs, blue for TTLs, gold for ROMs, purple for RAMs, silver for PLDs. This colour marking system is of great help when working on PCBs, it gives an immediate visual overview of what is what, it avoids missing out something important to dump, it saves a lot of time because I do not have to re-identify components when I look at a PCB after some time.

First thing we have to find the main CPU (maybe there are more than one!); it's usually the only chip whose size is remarkably different from all the others. We have to remember that 6502, Z80 and 68000 are the most used CPUs in arcade game history, learning by heart what they look like is fundamental. Obviously each chip sports some writing (markings), and looking that up on the Internet is always a good way to confirm what a chip is. For CPUs identification I would recommend CPU-World as a reference. Sound CPUs are usually found grouped together, close to the volume trimmer. Most common sound CPUs are M6295 (and clones) and all of the various YMxxxx. On some PCBs we will also find MCUs; they are like CPUs, but with an internal memory which is unfortunately read-protected 99% of the times. On some PCBs there will also be “support chips” like I/O chips, CRC controllers or Peripheral Interface Adapter; let's mark all of them green as well.

Second thing let's rule out all of the TTLs; they are easy to identify, because they are small (300mil) and marked 74xxx (in very old PCBs they can also be 54xxx), let's mark them blue. TTLs are usually the most common component, ranging from some tens to some hundreds of them on a single PCB. They are pure logical operators and are of very scarce interest for the dumper, apart from a few remarkable exceptions: 74s188, 287, 288, 370, 387, 471, 472, 473, 474, 570, 571, 572, 573. These are all PROMs, they have to be marked gold and are addressed here following.

Third thing is to correctly identify and colour mark ROMs. They come in a variety of forms: PROMs, EPROMs, EEPROMs, FlashROMs, MaskROMs. The most common are EPROMs, they sport a transparent window, they are 600mil, ranging from DIP24 to DIP42. In most PCBs they are socketed, and their extraction is very easy using a small flat screwdriver and slowly lifting them with small movements from BOTH sides to avoid any pin bending. MaskROMs are of the same size as EPROMs, but without the window, and very often they are soldered through the PCB. Removing them is a little bit more complicated, and the best suggestions for it can be found at Guru's site. EEPROMs and FlashROMs have different sizes, are usually SMD, and special adaptors are required to insert them into your programmer; adaptors could be rather expensive. Finally PROMs are small chips (300mil) usually soldered through and quite similar to TTLs, therefore learning by heart their specific markings and labels comes handy in many cases. A good reference is available here.

Fourth thing are RAMs, they come in many sizes, from long 300mil to thin 600mil to other (less common) sizes. Mark them out, because you will need to mention them in the documentation.

Fifth thing is to find out if any PLD is on your PCB. To understand what a PLD is, please refer to this very good article on wikipedia. PLDs always incorporate a “read protect” feature, in 95% of the case it's activated by the producer; until recently it was impossible to read them, a dumper simply gave them a try hoping to be lucky and get into the 5% which is not protected. Recently Charles McDonald found a way to read also a certain category of PLDs (the most common on arcade PCBs), so the above situation is changing.

Sixth and last thing is to identify all other components of the PCB. You will find resistors, trimmers, LEDs, audio parts, heat exchangers, connectors, jumpers, switches and a lot of other things. You will need to describe them in the documentation as well.

2- Components dumping -stub-

3- checking the dump -stub-