This article discusses in detail the Non-Posted Transaction (including Ordinary Read, Locked Read, and IO/Configuration Writes) and Posted Writes (including Memory Writers and Message Writes).

Non-Posted Transaction

o Ordinary Reads

The following figure shows an example of an Endpoint sending a Read Request to System Memory.

In this example, the Endpoint's read request passes through two Switches and then reaches its destination, Root. Root decodes the read-requested packet, identifies the address of the operation, latches the data, and sends the data to the Endpoint, the Completion packet containing the data, ClpD. It should be noted that PCIe allows a maximum data payload (Max Data Payload) of 4KB per packet, but in practice it is often necessary to send more than 4KB of data. Therefore, often one read request corresponds to multiple ClpDs, that is, more than 4 KB of data is divided into multiple packets to send. If an error is encountered, Root informs the corresponding Endpoint via the Completion package.

Note: The Root sends the Completion packet to the Endpoint sending the request. It searches for the corresponding Endpoint through the BDF information (Bus, Device and Function) in the Request packet. About BDF, it will be introduced in detail in the following article.

o Locked Reads

The Locked request is actually a way for PCIe to be set up to be compatible with the earlier PCI bus. For non-PCI compatible designs, the Locked operation is not allowed. And only the Root can initiate the Locked request operation, and the Endpoint cannot initiate the Locked request operation. The following figure shows a simple Locked Read request operation:

Locked Read is mainly used to support an operation called Atomic Read-Modify-Write, which is a high-priority and uninterruptible operation. It is mainly used for testing tasks such as link status (for PCI devices, PCIe devices are prohibited from using the Locked operation). In addition, the Locked operation uses the Target Memory Address to find the Legacy Endpoint (PCI device) rather than the BDF described earlier.

o IO/Configuration Writes

The following figure is an example of a Non-Posted IO write operation. Like Locked operations, IO operations are also for compatibility with earlier PCI devices and are not recommended for PCIe devices.

Posted Writes

o Memory Writes

As mentioned in the previous article, Memory writes in PCIe are all Posted, so Requester does not require Completion from the Completor. A simple Memory Writer example is shown below:

Therefore, Completion is not returned, so the Requester will not know when an error occurs. However, at this point, the Completer logs the error to the Log and sends the Root a Message containing the error message.

o Message Writes

Unlike several other types, Message supports multiple Routing methods. For example, Requester can send Message to a specified Completer, but regardless of whether the specified Completer is Root, Root will automatically receive Message sent from any Endpoint. In addition, when the Requester is Root, the Requester can also send messages to all Endpoints.

It must be said that the introduction of the Message mechanism helps the PCIe bus to eliminate many of the sideband signals in the PCI bus. Many PCI sideband signals used for interrupts, power management, and error reporting are implemented in Messages through PCIe.

Safety Controller

CSRME safety controller is developed for standard GB27607. By monitoring machine tool safety related equipment, the security of machine control system can meet the requirements of GB27607, and its security meets the requirements of ISO13849-1 (PLe) and IEC61508 (SIL3).

With rich interfaces, CSRME has limited programmable function. It can simultaneously replace many different types of safety control modules or safety PLCs, thus greatly simplifying the safety design of machine control systems and reducing cost.

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Jining KeLi Photoelectronic Industrial Co.,Ltd , https://www.sdkelien.com

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