Systems Integration and Test

In my company we have to integrate software, hardware, firmware, electrical, electronics, mechanical and human machine interface sub-systems to develop a simulator.
We do Star Integration (or also known as Spaghetti Integration is a process of integration of the systems where each system is interconnected to each of the remaining subsystems) and follow Product Integration Process Area of Capability Maturity Model® Integration (CMMI®).
Related Special Goals (SG) and Special Practices (SP) are as follows:
SG 1 Prepare for Product Integration
SP 1.1 Determine Integration Sequence
SP 1.2 Establish the Product Integration Environment
SP 1.3 Establish Product Integration Procedures and Criteria
SG 2 Ensure Interface Compatibility
SP 2.1 Review Interface Descriptions for Completeness
SP 2.2 Manage Interfaces
SG 3 Assemble Product Components and Deliver the Product
SP 3.1 Confirm Readiness of Product Components for Integration
SP 3.2 Assemble Product Components
SP 3.3 Evaluate Assembled Product Components
SP 3.4 Package and Deliver the Product or Product Component

You can also get information about the sub-practices at Software Engineering Institute.
According to my experience the most important sub-practice is
"Review interface data for completeness and ensure complete
coverage of all interfaces." When you integrate a system you have to define interfaces between sub-systems in complete coverage. To do this you'll need to begin searching interfaces at requirement analysis phase and document all requirements of interfaces in System Subsystem Requirements (SSR) Document. Later, in the Systems Design phase you'll identify all interfaces of your system.
So in the Systems Integration phase, you can define tests to verify the interfaces.
It's also important to document a "Systems Integration Plan".

Here's a template for "Systems Integration Plan":
1 SCOPE
1.1 Document Overview
1.2 Identification
1.3 System Overview
1.4 Abbreviations
2 REFERENCED DOCUMENTS
2.1 Company Reference Documents
2.2 Standards
3 GENERAL CONSTRAINTS
4 INTEGRATION STRATEGY
4.1 Simulator Components
4.1.1 Software
4.1.2 Human Machine Interface (HMI)
4.1.3 Mockup Mechanical
4.1.4 Electrics
4.1.5 Motion Platform
4.1.6 External Environment
4.2 Integration Sequence
4.2.1 Software – HMI
4.2.2 Software – Mockup
4.2.3 Software – Motion Platform
4.2.4 Software - Electrics
4.2.5 Mockup Mechanical – HMI Integration
4.2.6 Mockup – Motion Platform
4.2.7 Mockup – Electrics
4.2.8 HMI - Electrics Integration
5 INTEGRATION TESTS
6 INTEGRATION – TESTS TRACEABILITY MATRIX

There is another important point to those methods. That is effective communication of the teams. My experience is as follows:
As a systems integration specialist I have to deal with people from different disciplines. Integrating sub-systems really means that integrating people of Software, Mechanical, Electronics, and etc. Each discipline has its own point of view for the integration of the system. So a systems integrator has to establish a good communication network between those people.