What is the topology of the motherboard memory bus

When overclocking memory, every nuance matters, even the subtleties of the layout of a particular motherboard. Let’s see what topologies are, how the channel length and the quality of the impedance matching of the communication line affect. Let’s consider which configuration of RAM is better to choose in specific conditions.

Types of motherboard topologies
Topology is a scheme of interconnection of functional units using communication channels. With regard to computer technology, there are two types of connection between the central processor and RAM – in the form of the letter “T” and Daisy Chain.

T-topology provides equivalent conditions for the exchange of information between the central processor and each pair of RAM modules. Since they are connected in parallel, this arrangement is better optimized for four-bar operation. With two RAM modules, the overclocking result will be worse.

Most users use exactly two sticks of memory, so the scheme is not widely used. It is used in premium motherboards where a large amount of RAM is important, such as the ASRock X570 Extreme4.
Daisy Chain (translated as “chain of daisies”) means a serial connection of modules. The circuit is better suited for two memory sticks. In this case, they must be installed in the “first” slots A2 and B2, that is, in the communication channels closest to the processor. If you connect all four modules using non-priority slots A1 and B1, the frequency performance will deteriorate. The topology is very popular and is used in most motherboards.

Read an example of overclocking AMD Ryzen 9 3900X processor on GIGABYTE X570 AORUS PRO with Daisy Chain topology

A special case of Daisy Chain, when there are no two additional slots (not because of economy, but to achieve maximum frequencies) is called the I-topology .

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This scheme implies the maximum possible overclocking of RAM. Due to the absence of “tails” in the form of non-priority slots, the best matching of the processor communication channel with two RAM modules is ensured. An example of such a motherboard is ASUS ROG Strix X570-I Gaming.

hat in budget boards, the presence of only two slots is due to the savings in production. This dual slot has nothing to do with gaming solutions designed for extreme overclocking.

How does the length of the communication channel affect?
As you know, the speed of propagation of an electrical signal in a conductor is equal to the speed of light. Let’s conventionally take the length of the PCB track, which goes from the processor to the RAM and back, equal to 10 centimeters. Thus, the time it takes for the signal to cross this section is 0.33 nanoseconds. This time is spent only on overcoming the length of the conductor, without taking into account the time for internal processes. That is, the RAM in at least 0.16 nanoseconds will receive the signal transmitted by the processor. And after another 0.16 nanoseconds, the CPU will receive a response from the RAM.

The delay imposes a limitation on the maximum number of clock cycles per unit of time when the RAM – processor link is working. It is not difficult to recalculate it into a frequency, which in this case will be 3 GHz (and the effective memory frequency, that is, with twice the amount of data transmitted per clock, is 6 GHz).

Thus, the longer the communication channel, the lower the maximum RAM frequency. In the T-topology, the track spacing from the CPU to the memory is the same, but the total channel length is longer than that of the Daisy Chain.

 

How the negotiation of the communication channel affects
The operating memory frequency corresponds to the radio frequency range: the exchange signal of the processor and RAM is, in fact, a radio signal. Therefore, the conditions for the transfer of this signal in the conductor completely obey the laws of propagation of radio waves.

For reliable transmission of a radio signal, without losses and multiple reflections from inhomogeneities of the data transmission path, it is necessary that the characteristic impedance of the signal source (processor) and the communication channel and load (RAM) are equal. In the case of a mismatch of wave impedances in any section, non-uniformity arises. The radio signal partly passes further to the consumer and partly is reflected back towards the source. There is an overlap on the subsequent signal, which leads to distortions and errors in the processing of the RAM.

 

Obviously, with an undistorted signal, the overclocked RAM works at its maximum stable frequency values. With the appearance of distortions due to mismatch of wave impedances, a decrease in the stable operating frequency occurs. This happens if you do not follow the manufacturer’s recommendations – for example, use only two RAM modules in a motherboard with a T-topology.