Dell Precision T7400 Memory issue

You cant mix ranks , speed  and density and you shouldnt mix vendors for fully buffered dimms.

This computer supports a maximum of 32 GB of memory when eight 4-GB DIMMs are installed. all slots should contain the same size/type dual channel memory. Pairs are required you cant use 3 you can use 1 or 2 or 4 or 6 or 8

1 dimm works because this is failsafe testing mode.

Ian_90,

Below is the memory information for this computer.

Memory

The memory controller hub (MCH) provides:
•Four channels of fully buffered DIMM (FBD) memory support
•Up to four dual ranked FBD DDR2 DIMMs in each channel
•Up to 16 DIMMs for a maximum memory size of 128 GB physical memory
•Supports DDR2 PC2-5300F (667 MHz) and PC2-6400F (800 MHz) FBD
•Branch channels are paired together in lock step to match front side bus (FSB) bandwidth requirements
•A special single channel, single DIMM operation mode is supported for failed DIMM isolation purposes.

The memory subsystem consists of four fully-buffered DIMM (FBD) memory channels. In the Dell Precision™ Workstation T7400 (without memory expansion risers) up to two DIMMs can be connected to each FBD channel, for a total of eight DIMMs. A Dell Precision Workstation T7400 with memory expansion risers supports up to four DIMMs on each FBD channel, for a total of sixteen DIMMs.

Memory Slots

There are 8 memory slots on the system board. The slots are numbered DIMM1 through DIMM8. DIMM1 is closest to the PCI slots and it is numbered through DIMM8 towards the outside of the system board.

Dell Precision Workstation T7400 supports only fully buffered DDR2 DIMMs, referred to as FBDs or FBD. Fully buffered DDR2 DIMMs consist of a set of normal DDR2 memory devices plus what is known as an Advanced Memory Buffer (AMB). This AMB communicates via a narrow high speed frame-oriented interface (similar to PCI-E) to the MCH and other DIMMs in its particular channel. The AMB also translates the serial memory accesses into DDR2 signals for communication with the DRAM devices.

The four FBD channels are organized into two branches of two channels per branch. The two channels on each branch operate in lock step to increase FBD bandwidth. A branch transfers 16 bytes of payload/frame on Southboard lanes and 32 bytes of payload/frame on Northbound lanes.

Dell Precision Workstation T7400 supports two speeds of DIMMs. The slower speed is PC2-5300F, which corresponds to 667 MHz DDR2 devices and an FBD serial channel speed of 4.0 GHz. This speed matches the bandwidth of CPUs with FSB frequency of 1333 MT/s The faster speed is PC2-6400F, which corresponds to 800 MHz DDR2 and an FBD serial channel speed of 4.8 GHz. This speed matches the bandwidth of CPUs with FSB frequency of 1600 MT/s.

The memory subsystem supports DRAM technologies of 256 Mb, 512 Mb, 1 Gb, and 2Gb with organizations of x4 and x8. This gives supported DIMM sizes of 256 MB, 512 MB, 1 GB, 2 GB, 4 GB and 8GB. Dell Precision Workstation T7400 with eight DIMM slots are able to support up to 32 GB of memory. Dell Precision Workstation T7400 with optional memory expansion risers have sixteen slots and are able to support up to 128 GB of memory.

Memory Population Rules

To achieve full bandwidth operation, memory configurations should consist of sets of four identically matched DIMMs to allow Quad-Channel operation. The sets of DIMMs must be identical with respect to capacity, rank, and organization (x4 or x8). Speed and CAS latency don't necessarily need to match, but the slowest speed DIMM and highest CAS latency will set the operating mode of the system.

For eight slot configurations:
•A single DIMM will work in DIMM1 or DIMM3 slot. This will be a reduced performance mode. BIOS will stop with an "F1/F2" message indicating this is a debug only mode. This mode is for use by tech-support to test a customer's DIMMs individually in the case of a reported memory failure.
•Configurations of two matched DIMMs are supported using DIMM1 and DIMM2 slots. This configuration provides half the memory bandwidth, but will be allowed to boot normally without any BIOS messages. This support is specifically to enable a low cost 1 GB (2 x 512MB) shipping configuration.
•Configurations of four matched DIMMs must use DIMM1, DIMM2, DIMM3, and DIMM4 slots. These configurations allow full bandwidth.
•Eight DIMM configurations use all eight DIMM slots and must consist of two matched sets of four DIMMs. These configurations have the same full bandwidth as four DIMM configurations.
•Additional non-shipping (but valid) configurations can be made by installing matched pairs of DIMMs in slots DIMM1/DIMM2, DIMM1/DIMM3, DIMM3/DIMM4, DIMM1/DIMM2 & DIMM5/DIMM6 and DIMM3/DIMM4 & DIMM7/DIMM8. These will result in dual channel operation which is lower bandwidth that quad-channel. We do not ship these configs, but they are valid and no error message is displayed.
•For thermal reasons, only DIMMs with a 'Full heat spreader' FLHS are supported. DIMMs with a heat sink only on the AMB will overheat and are not supported.

Memory Error Messages

In the event of a memory issue, the BIOS will attempt to configuration video and display a text message. A listing of possible messages are shown in the table below.

If the memory is too non-functional for the BIOS to display a text message, a diagnostic LED code will be displayed.

Stop Type

Error Message

Description

F1/F2 Alert! Memory configuration mismatch. Please enter Setup for Memory Info details. Memory is installed in any config that doesn't at least have matched pairs.
System Halt Alert! Memory Risers must be installed as a set of four. Memory Risers are only supported if all four are installed together.
F1/F2 Alert! Uncorrectable memory error previously detected in DIMM x or y. On the previous boot, the ECC mechanism detected, but was unable to correct a memory error that occur in either DIMM x or DIMM y.
F1/F2 Alert! Operating in single channel debug mode. Please populate memory in matched set for optimal operation. Booting with only a single DIMM installed in DIMM1. Customer can press "F1" and boot normally, but bandwidth is reduced.
F1/F2 Alert! Memory Built-in Self Test fails in DIMM x or y. An error was detected during memory testing done during POST. It occurred in either DIMM x or DIMM y.
System Halt Alert! Invalid memory installed. DIMMs installed on Memory Risers must have full DIMM heat spreaders. Due to thermal limitations, only DIMMs with full heat spreader are allowed on memory risers.
System Halt Alert! Invalid memory installed. 667 MHz DIMMs must have full DIMM heat spreaders. Due to thermal limiations, all 667MHz must have full heat spreaders.

Memory Risers

The memory expansion risers (RISER_1, 2, 3, and 4) plug directly into the memory slots on the system board (DIMM_1, 2, 3, and 4 respectively). They increase the available memory slots from eight to sixteen. The memory expansion risers are not interchangeable, and each will only work with its correspondingly numbered slot. They come assembled in pairs with RISER_1 and RISER_2 fastened together, and RISER_3 and RISER_4 together.

NOTE:
Just as each of the first four memory slots on the system board do, each riser represents one memory channel.

Memory Population Restrictions

For sixteen slot configurations using memory expansion risers:
•There are four unique memory risers that must be installed together into system board DIMM1, DIMM2, DIMM3, DIMM4 slots. (RISER1 plugs into DIMM1 slot, RISER2 into DIMM2, etc.)
•When risers are installed, nothing may be installed in system board slots DIMM5 - DIMM8.
•Each of the four risers provides four DIMM slots, resulting in a total of sixteen slots available.
•Configurations of four matched DIMMs need to be installed one per riser starting with the lower-most riser slot (labeled DIMM1). For example, a set of four matched would be installed in RISER1-DIMM1, RISER2-DIMM1, RISER3-DIMM1, and RISER4-DIMM1. All four risers must be spanned with the matched set of DIMMs in order to achieve full bandwidth operation. Each riser is a single FBD channel.
•Configurations of eight, twelve and sixteen DIMMs must be composed of sets of four matched DIMMs, installed with one DIMM of each set on each riser. This maintains the required spanning to get full four channel bandwidth.
•As on the system board, there exist valid non-shipping configurations composed of matched pairs of DIMMs. The matching is across risers. For example: RISER1-DIMM1/RISER2-DIMM1, RISER3-DIMM1/RISER4-DIMM1, RISER1-DIMM1/RISER3-DIMM1, etc. Again, no error message is displayed for these even though they are non-shipping configurations.
•Each riser should be populated identically.
•For thermal reasons, only DIMMs with a 'Full heat spreader' FLHS are supported. DIMMs with a heat sink only on the AMB will overheat and are not supported.

Memory Slots

There are 4 memory slots on each memory riser. The slots are numbered DIMM_1 through DIMM_4. DIMM_1 is closest to the bottom of the riser and it is numbered through DIMM_4 towards the top.

this was a double post, ignore this (sorry i'm new here)

So is there a way to get 11GB of usable memory? These are the memory modules I have:

4x1GB (Hynix)

3x1GB (Unkown vendor)

1x4GB (Hynix)

They're all 667 MHz.

I also tried to put the all the 1GB modules in DIMM 1-4 and the 4GB module in DIMM 5, but it is only showing 4GB.

@SpeedStep

I think I finally understand it, it only shows 6GB because there are 3 pairs of 1GB installed. Also vendor doesn't matter that much, 1 pair is from another vendor and it works just fine. I didn't understand anything at first because I'm used to work with regular memory, not buffered memory.

Thanks a lot for your help!