🧠 Memory Guide • Server & Workstation
ECC Registered (RDIMM) vs ECC Unbuffered (UDIMM)
A visual, no‑nonsense guide to understand buffering, ECC, compatibility, and why UDIMM motherboards can’t read RDIMM modules.
Physical differences you can spot
- Extra chip on RDIMM: the register (often centered). UDIMM lacks it.
- More DRAM packages possible on RDIMM without overloading the controller.
- Labeling: look for “RDIMM / Registered / R‑ECC” vs “UDIMM / Unbuffered / U‑ECC”.
- Look for the letter "R" in RDIMM and the letter "U" for UDIMM.
UDIMM (ECC Unbuffered)
Best for low‑to‑mid capacities
- Lower latency (no register stage)
- Lower cost, simpler design
- Electrical load increases with more chips/modules
- Common in workstations / entry servers
RDIMM (ECC Registered/Buffered)
Best for scaling up
- Register chip buffers command/address signals
- Reduces load on the memory controller → supports more DIMMs/ranks
- Small latency penalty (≈ one extra clock)
- Standard in servers and high‑capacity systems
Why a UDIMM motherboard can’t read RDIMM
UDIMM board expects… Unbuffered
- ✓ Direct drive to DRAM chips (no register present)
- ✓ Timing and training tuned for direct path
- ✓ Simpler SPD / wiring assumptions
RDIMM actually has… Buffered
- ✗ A register between controller & chips → signals don’t match expectations
- ✗ Different training/latency model (extra stage)
- ✗ Different electrical loading & SPD flags → firmware rejects it
