Document ID: TOS-DIAG-REF-003 Revision: 1.0 Author: ThisOldScot
Section_1: Overview#
In the Borders Lab, when an Oscilloscope (TOS-DIAG-REF-001) confirms a signal is present, but the system still hangs, we deploy the Logic Analyzer. While a scope shows the “shape” of electricity, the analyzer decodes the “meaning” of the data.
This tool allows us to monitor 8, 16, or 32 channels simultaneously, capturing the high-speed handshake between the CPU, BIOS ROM, and System RAM. If a single bit on the Address Bus is stuck Low due to a microscopic trace break, the Logic Analyzer is the only instrument that will reveal the specific coordinate of the failure.
Section_2: Essential Equipment & Setup#
| Component | Specification | Function |
|---|---|---|
| Logic Analyzer | 100MHz+ Sample Rate | Capturing the 24MHz+ bus speeds of 16/32-bit consoles. |
| IC Hooks / Grabbers | High-density / Micro | Attaching to fine-pitch SMD pins without shorting. |
| Software | PulseView / Sigrok | Open-source suite for protocol decoding. |
| Fly-Leads | Shielded | Reducing crosstalk during high-speed data capture. |
Section_3: The Bus Sniffing Protocol#
STAGE_3: 01 // Mapping the Address Bus (A0-A23)#
The Address Bus is the set of lines the CPU uses to “point” to a specific location in memory.
- The Test: Attach probes to the Address lines on the BIOS/ROM chip.
- Analysis: Upon power-on, you should see a rapid binary count. If lines A0-A7 are toggling but A8 is permanently Low (0V), the CPU cannot access memory beyond the first 256 bytes, causing a crash.
STAGE_3: 02 // Monitoring the Chip Select (CS / CE)#
The Chip Select line is the “Hey, you!” signal from the CPU to a specific peripheral.
- The Logic: A CPU might talk to RAM, the Video Controller, or the BIOS. It pulls the
/CSline Low for the specific chip it wants to read. - Diagnosis: If
/CSon the BIOS never pulses Low, the CPU is “hung” internally and isn’t even attempting to look for software instructions.
STAGE_3: 03 // Protocol Decoding (The “English” Translation)#
Using software like PulseView, we can apply “Decoders” to the captured bits.
- Example: On a Neo-Geo or Mega Drive, we can decode the 68000 Instruction stream.
- The Clue: The analyzer can show us the last instruction sent before the system crashed (e.g., an “Illegal Instruction” error), pointing directly to a corrupted ROM or a faulty RAM chip.
Section_4: Identifying “Floating” Pins#
A common failure in vintage consoles is an “Open” pin—a solder joint that looks fine but has no electrical contact.
- Forensic Clue: In the analyzer software, a floating pin often shows up as random, high-frequency “noise” or a signal that mirrors its neighbor.
- The Fix: Reflow the identified pin using the SOP-8/QFP Reflow Protocol (TOS-TOOL-011).
Section_5: Technical Warning#
Safety_Warning
INPUT CAPACITANCE ALERT: Logic analyzer leads add “capacitance” to the circuit. On sensitive high-speed buses (like the N64 RDRAM), attaching too many probes can actually crash a working system by distorting the signal timing. Always keep leads as short as possible and probe only the necessary lines.
Section_6: Practical Application: PS1 “Sony Logo” Hang#
If a PlayStation 1 displays the Sony logo but hangs before the “Black Logo” (indicating a BIOS-to-CD handoff failure):
- Attach the analyzer to the Parallel I/O Port pins (which mirror the main bus).
- Capture the data stream during the transition.
- Analysis: If the analyzer shows the CPU requesting data from the CD controller but receiving
0xFF(null data) in return, you have confirmed a communication break on the Data Bus between the CD-DSP and the CPU.