PC doesn’t POST when a specific monitor is plugged into DP1 of GPU with CSM Support disabled.

Understanding POST Failures When Connecting a Specific Monitor to Your GPU’s DisplayPort: A Case Study

Introduction

In the pursuit of upgrading system security compliance through TPM 2.0 and Secure Boot, many PC builders often encounter compatibility challenges linked to BIOS settings and display hardware. A particularly perplexing issue occurs when disabling CSM (Compatibility Support Module) in BIOS leads to system POST failures depending on how and where the monitor is connected. This article delves into a real-world scenario involving such a configuration and explores potential causes and troubleshooting steps.

The Scenario

A user with the following hardware configuration:

Motherboard: X570 Aorus Elite
CPU: AMD Ryzen 7 5800X3D
GPU: EVGA GeForce RTX 3090 FTW3
Monitors: Main monitor connected via DisplayPort (DP1), and secondary monitor connected via DP2
Problematic Monitor: ViewSonic VX2758-2KP-MHD

Issue Overview

The user’s objective was to enable TPM 2.0 and Secure Boot, which required disabling CSM support in BIOS for compatibility with Windows 11 and other security features. However, this adjustment introduced an unusual POST failure pattern:

When the main monitor is connected to DP1, the system fails to POST entirely:
No BIOS or OS load
No response to keyboard input
System appears to hang during POST indefinitely
Conversely, if the main monitor is connected to DP2, with the secondary monitor on DP1, the system POSTs normally:
Both displays turn on
Operating system loads without issue

This inconsistent behavior suggests a nuanced interplay between GPU BIOS settings, monitor hardware, and display connection protocols.

Potential Causes

DisplayPort Handshake and Hotplug Detection

Modern GPUs perform a handshake with connected displays during POST to establish display capabilities. Certain monitors may not complete this handshake successfully during early POST stages, particularly if CSM is disabled, which shifts GPU operation to UEFI mode and modifies the graphics initialization process.

Monitor Compatibility and EDID Reporting

The problematic ViewSonic monitor may have specific EDID (Extended Display Identification Data) responses or handshake peculiarities that trigger failure when plugged into a specific port. Different DisplayPort connectors on the GPU can sometimes behave differently due to internal configuration or firmware nuances.