Diagnostic Serial Console

The Macintosh Plus, SE and SE/30 feature a built-in diagnostic console accessible via a serial connection, which can be used for detailed hardware troubleshooting. Originally designed for use with specialized Apple diagnostic tools, this diagnostic mode can also be accessed via a standard serial connection using software such as PuTTY or similar serial terminal programs. Adrian Black's spreadsheet on this topic provides the most useful breakdown of how to use the diagnostic console Macintosh Diagnostics / Test Manager

Connecting to the Diagnostic Console

Connect your Macintosh to another computer using a serial cable (via the modem port):

• Set your serial terminal (such as PuTTY) to the following:
  • Baud Rate: 9600
  • Data bits: 8
  • Stop bits: 2
  • Parity: None
• Once connected, restart your Macintosh while holding the interrupt switch or issuing an interrupt command to enter diagnostic mode.

Commands & Tests

Below are detailed descriptions and commands for running specific diagnostic tests:

Preprogrammed Diagnostic Test Commands

Command	Test Description
*T	Runs preprogrammed diagnostic tests. Use with specific codes to run targeted tests.
*T0000	No operation (null test)
*T0001	ROM Checksum – Verifies ROM integrity. Errors indicate corrupted or damaged ROM.
*T0002	Small RAM Chunk Test – Checks initial RAM area for immediate failures.
*T0003	Full RAM Test (Bank B) – Tests full Bank B RAM after small chunk passes.
*T0004	Full RAM Test (Bank A) – Checks the second RAM bank thoroughly.
*T0005	Address Line Test – Verifies RAM addressing and unique addressing capability.
*T0006	VIA1 Access Test – Checks the proper operation of VIA1 chip.
*T0007	VIA2 Access Test (Mac II only) – Checks VIA2 chip operation.
*T0008	Front Desk Bus Test – Verifies access to Front Desk Bus components.
*T0008	MMU Access Test – Tests access to the Memory Management Unit.
*T0009	NuBus Access Test – Ensures NuBus communication is functional.
*T000A	SCSI Chip Test – Confirms proper operation of SCSI controller.
*T000B	IWM Chip Test – Verifies floppy drive controller functionality.
*T000C	SCC Chip Test – Tests Serial Communications Controller for proper operation.
*T000D	Data Bus Test – Validates data bus functionality, identifying bit faults.

Custom Non-Critical Tests

Non-critical tests use a different format:

Format: *TXXY

• XX = Hexadecimal test number
• Y = Number of repetitions

Example: *T841 runs test 0x84 once.

Custom Non-Critical Diagnostic Tests

Hex Code	Test Description
80	Video RAM Test – Tests VRAM operation (Macintosh II).
81	Video Interface Test – Checks functionality of video hardware interface.
82	VIA Test – Comprehensive VIA chips test.
83	SCSI General Test – Tests general SCSI functionality.
84-86	SCC (Serial Communications Controller) Tests – Checks serial communications.
87	SCSI General Test – General SCSI device communication test.
88	Sound Test – Verifies audio hardware function.

Interpreting Response Codes

The console returns hexadecimal response codes:

Typical Response Codes

Response Code	Meaning
020004030000	Test-specific failure indication (consult specific test for details).
020004030000	Data Bus Error accessing RAM.
0F000D	NMI button pressed during boot (normal indication).

RAM Chip Identification (Subcode Explanation)

For memory tests, subcodes identify specific faulty chips:

RAM Chip Location & Subcode Identification

Data Bit	Chip Location	Subcode Bits (Hexadecimal)
0	F5	0001
1	F6	0002
2	F7	0004
3	F8	0008
4	F9	0010
5	F10	0020
6	F11	0040
7	F12	0080
8	G5	0100
9	G6	0200
10	G7	0400
11	G8	0800
12	G9	1000
13	G10	2000
14	G11	4000
15	G12	8000

Here is an example of testing 4MB of memory

• A
• R
• 4
• 3
• 000000000
• 0003FFFFC
• T000600010001

Once we get *T back...

• R

It should return all 0s if good.

Important Notes

• Not all diagnostic tests work on every model; compatibility varies with ROM version.
• SE/30 and other 256k ROM Macs do not support certain diagnostic commands listed here.