Video Board. JTAG Bus Order. Boot up. Boundary Scan

crackedMagnet’s

Sega Model 3 Step 2. 0

Repair Guide

Video Board

JTAG Bus Order

This order was worked out from tracing tracks to and from the video ICs (both frame buffer and depth memory). Then I captured the JTAG bus on bootup. The datasheet for the non BGA ICs indicates that they start in bypass mode for the JTAG as they have no ID Code, meaning they will just show up as a single bit with the value 0.

Here is a table of the ICs on the video board JTAG bus in order from TDO (output) to TDI (input). So these bits are in the order they flow out of the TDO. Not sure if the ID codes need to be reversed to get their proper values. I should also note that the order of IC76, IC77 and IC90 might be wrong, as they are BGA’s and its difficult to trace the tracks between them.

ID Code	IC
01000001 01101100 01110000 01010111	IC90
01000001 01101100 00110000 01010111	IC76
00110001 01101100 01000000 01010111	IC77
01000001 01101100 01010000 01010111	IC78
	IC62
00110001 01101100 01100000 01010111	IC92
00110001 01101100 01100000 01010111	IC79
	IC85
	IC91
	IC99
01000001 01101100 01010000 01010111	IC178
	IC162
00110001 01101100 01100000 01010111	IC192
00110001 01101100 01100000 01010111	IC197
	IC185s
	IC191s
	IC199s

Boot up

If there is a fault with the JTAG bus, like one of the registers being stuck high or low, the system will not boot. If I recall correctly two of the “frame” leds flash, thought I had a photo of that but can’t find it. On startup there it seems to go through a partial scan of ICs then a full scan. I suspect it's trying to detect if there is one or two video processor sets connected.

Boundary Scan

The boundary scan is a jtag function that allows the pins on an IC to be read or set from the jtag bus. The idea is you can set a pin on one IC high and read it from the IC it should be connected to in order to see if its connected. It’s particularly important with BGAs as you can’t access the contacts so testing them directly isn’t very practical.

The boundary scan option is not available on all model 3 step 2 games. Sega Rally 2 doesn’t have this option (or any ram tests by the looks). Daytona 2 Power Edition does and I believe Ambulance Call also does. There may be many others that have the boundary scan option, I just haven’t found out which. If there is a game that has the boundary scan option and doesn’t require a security board let me know and I’ll add that to this guide as it might allow people to download and program roms that could help them diagnose their boards.

Don’t assume boundary scan errors mean there is a problem!

For some reason which I haven’t worked out yet video boards can have 10, 000 or so boundary scan errors when they seem to be working perfectly. I’ve also got at least one video board that comes up with no errors. I suspect this might be some difference between the step 2. 0 and step 2. 1 boards. At some point I should probably get ID codes for each of the ICs from working boards with and without the boundary scan errors and see if there are any differences.

Where I’ve found the boundary scan most useful is when I’ve replaced one of the ram ICs and its still comes up as failing. Given I’m expecting a problem with that IC (ie my soldering), it can point to a pin or two, which I can confirm with a meter is not connected.

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