Asynchronous communication method of the bus
Asynchronous communication overcomes the shortcomings of synchronous copper whiskers, allowing the speed of each module is not consistent, giving the designer full flexibility and choice. It does not have a public clock standard, does not require all parts of the strict uniform operation time, but the answer mode (also known as handshake), that is, when the master module sends out a request signal, has been waiting for the module from the feedback back to the "response" signal before the start of communication. This requires the addition of two new answers between the master and slave modules.
Asynchronous communication, also known as answering communication, is a type of communication based on an answering or interlocking mechanism. That is, the moment at which the latter event appears on the bus depends on the appearance of the previous event. In this type of system, there is no need for a uniform common clock signal, the length of the bus cycle is variable, and no response time is imposed on the functional components, thus allowing both fast and slow functional components to be connected to the same bus, but this comes at the expense of increasing the complexity and cost of the bus.
Asynchronous communication can be categorized into three types according to whether the answer signals are interlocked or not, i.e., whether the establishment and withdrawal of the request and answer signals are dependent on each other: non-interlocked, semi-interlocked, and fully interlocked communication.
Non-interlocking communication
Figure 1 Non-interlocked communication
After the master module sends out a request signal, it does not have to wait to receive the answer signal from the slave module, but after a period of time, after confirming that the slave module has received the request signal, it will revoke its request signal; after receiving the request signal, the slave module sends out an answer signal when the conditions permit, and after a period of time (the setting of this period of time is different for different devices), after confirming that the master module has received the answer signal, it will revoke the answer signal automatically. Answer signal. It can be seen that there is no interlocking relationship between the two sides of the communication.
For example, the COU writes information to the main memory thatCPUTo give address performance good, write command as well as write data successively, i.e., this method is used.
Semi-interlocked communication
Figure 2 Semi-interlocked communication
The master module sends out a request signal and must wait until it receives an answer signal from the slave module before revoking its request signal, or interlocking relationship; while the slave module sends out an answer signal after receiving the request signal, but does not have to wait to be informed that the arachnid's request has been revoked, but rather automatically revokes its answer signal after a period of time, without interlocking relationship. Since one side has an interlocking relationship and one side does not, it is called the semi-interlocking method.
For example, in a multi-host system, a particular CPU needs access to theshared storageWhen the memory is accessed by all CPUs, the CPU is not allowed to receive an answer signal that the memory is not occupied after issuing a command to access the memory in order to actually perform the access operation.
Fully interlocked communications
Figure 3 Full Interlock Communication
The master module sends out a request signal and must wait for the slave module to reply before withdrawing its request signal; the slave module sends out an answer signal and must wait for the master module to know that the request signal has been withdrawn before withdrawing its answer signal. There is interlocking relationship between two transmitters, so it is called full interlocking method.
For example, in network communications, full interlocking is used by the communicating parties.
Asynchronous communication can be used for parallel or serial transmission. Asynchronous parallel transmission is shown in Figure 4, where "Ready" and "Strobe" are the contact signals. In asynchronous serial communication, there is no synchronization clock, and there is no need to transmit a synchronization signal for data transmission. In order to confirm the character being transmitted, the agreed character format is: 1 start bit (low), 5 to 8 data bits (such as ASCI code for 7 bits), 1 parity bit (for error checking), 1 or 1.5 or 2 end bits (high). The start bit is immediately followed by the lowest bit of the character to be transmitted, and each character ends with a high level termination bit. The start bit to the end bit constitutes a frame, and the interval between two frames can be of any length.
Figure 4 Asynchronous parallel "answer" contacts