Culling memory. Video memory

Culling memory

The memory test reports the culling memory as faults with IC68, IC69, IC72 and IC73:

As with the polygon memory these IC numbers refer to the step 1. 5 board IC numbers:

So it appears IC68, IC72 refer to IC10 and IC69, IC73 refer to IC11.

One note I will add about replacing these is that if some of the solder joints aren’t connected the stack won’t boot. In my case it was some of pins in the corners. So if you replace one and it won’t boot afterward, check your joints.

In terms of what visual issues you get when these ICs aren’t working… I’m not entirely sure as I had replaced the polygon memory right before doing these ones so didn’t run it for long. I vaguely recall it might have been missing polygons, so you can see through bits of cars etc.

Video memory

These report in the memory test as either IC85, IC91, IC99 or IC185s, IC195s, IC199s:

The purpose of this memory is to store the current video frame. The video frame is able to be read of out the VID pins while being updated asynchronously from the video processors. It looks like ICs are grouped in 2 sets of 3. One set for each of the video processors. Given the changes in video output from the faults I’ve experienced it looks like the 2 video processors process alternate lines. And I suspect in each set of 3, one is for red, one green and one blue. I’m not sure which is which however.

One quite odd thing about they way these are used is the fact they tied the data lines together. PALU_DQ0 is connected to PALU_DQ15, PALU_DQ1 -> PALU_DQ16, PALU_DQ2 -> PALU_DQ17 etc. This effectively halves the capacity of the ram chip. There maybe some reason related to the way they read the video back of the chip that makes this necessary.

IC91 and IC199s are Mitsubishi M5M410092BRF chips, the rest are Mitsubishi M5M410092BFP. The only difference between the two types is one has its pins clockwise and the other counter clockwise. Well there is one other difference and that is price.

I bought my M5M410092BFP for about $5US each and M5M410092BRF for about $10US each. I ended up having to get them off alibaba as I couldn’t find them anywhere else.

Given the price tag replacing a set of 3 would cost about $20US, so it might be worth doing a boundary scan and a quick check of the joints on any pins that show up.

I should also note the memory test doesn’t seem to detect problems with the video output on these ICs.

Anyway here’s an example of what it can look like when one of these ICs has failed:

Note the distinct horizontal lines.

Here’s another example:

Again the horizontal lines, but also the obvious colour distortions.

It’s worth noting that a complete failure of one of these ICs would cause the stack not to boot. In these cases it’s probably necessary to put a logic sniffer on the JTAG bus to see at which IC the ID Codes start to go wrong.

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