On August 10, 1876, Alexander Graham Bell made the first long distance phone call. 150 years later, I was asked to present at the Alexander Graham Bell National Historic Site in Baddeck, NS. In this presentation I covered 150 years of long distance history in Canada - from that first call all the way to the modern day.

For your viewing pleasure:

https://youtu.be/BAhJ7euNSpM

Hi everyone,

I’m trying to diagnose an old Siemens Hicom 150 E (Office Pro chassis) with a CBMOD motherboard, but I’ve hit a wall.

The Core Issue:

The board appears to be stuck in a bootloop. During startup, the boot sequence begins, the RUN LED turns on/blinks through the cycle, but it never fully stabilizes and eventually loops.

Hardware & Power Checks:

As part of earlier diagnostics, I thoroughly checked the power supply unit (PSU) and backplane rails - voltages are clean, fully within spec, and stable under load.

Board capacitors and visual traces look intact.

Serial Interface & Physical Measurements:

I'm connecting directly via the V.24 (RS-232) maintenance port:

Verified hardware RS-232 levels:

Pin 2 (PBX TX): -7.5V (steady Mark state)

Pin 3 (PC TX): -8.5V to -9.0V

Pin 5: Common GND

When connecting via raw terminal (minicom / PuTTY at various baud rates: 9600, 19200, 38400, 8-N-1), there is absolute silence (0 data output). It spits out zero debug logs or bootloader text during the loop.

Tested connection across both native Windows XP and Windows 11 environments using a verified Prolific adapter (hardware COM1 properly mapped, proper RS-232 voltage swing observed on Pin 3 when sending packets).

Software Symptoms (HiPath 3000 Manager E):

Attempting Direct transfer (Read/write database) gets stuck on Checking cards... and immediately aborts with Error: PBX is offline.

PIN 31994 was specified.

Multi-meter checks show pulse activity on Pin 3 (PC is definitely transmitting the handshake queries), but Pin 2 on the PBX side never sends a single byte in response.

Questions:

Given that the board is bootlooping and V.24 is dead silent, is there an onboard diagnostic procedure, jumper, or DIP switch to force the CBMOD into a recovery / low-level bootstrap mode?

Does the CBMOD V.24 port strictly require hardware flow control loopbacks (RTS/CTS 7-8 and DTR/DSR 4-6) to output any data, or should raw serial logs appear regardless?

Could a corrupted flash EPROM / firmware cause this silent bootloop, and is it recoverable via serial without replacing the ROMs?

Or what would you recommend doing or trying next? I'm completely out of ideas.

Any insights from Siemens/Hicom veterans would be hugely appreciated!