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What is Serial asynchronous communication ?

Serial asynchronous communication is a common communication method between the host and external hardware devices. It can be transmitted in both directions. For example, the data received by the satellite signal receiving board is transmitted to the computer for processing. Serial asynchronous communication is mainly used for processing.


There is no need for synchronous clock signals in the communication process of serial asynchronous communication. Sometimes the baud rate must be agreed between the devices, and sometimes there is no agreement, and the hardware cost is low.


In asynchronous communication, there are two important indicators in asynchronous communication: character frame format and baud rate. Data is usually transmitted in character frames in units of characters or bytes.


The character frame is sent frame by frame by the sender, and received frame by frame by the receiving device through the transmission line. The sending end and the receiving end can control the sending and receiving of data by their respective clocks, and the two clock sources are independent of each other and not synchronized with each other.


When the receiving end detects the low-level logic "0" sent over the transmission line (ie, the start bit of the character frame), it determines that the sending end has started to send data. Whenever the receiving end receives the stop bit in the character frame, it knows A frame of characters has been sent.


A typical frame format in asynchronous communication is: 1 start bit, 7 (or 8) data bits, 1 parity bit, and 2 stop bits.


Common bus types for serial asynchronous communication




One of the communication interfaces on the personal computer, an asynchronous transmission standard interface established by the Electronic Industries Association (EIA). Usually the RS-232 interface comes in the form of 9 pins (DB-9) or 25 pins (DB-25).




RS485 is a standard that defines the electrical characteristics of drivers and receivers in balanced digital multipoint systems, defined by the Telecommunications Industry Association and the Electronics Industry Alliance.




It is the abbreviation of Universal Serial Bus (Universal Serial Bus) in English, and its Chinese abbreviation is "pass serial bus". It is an external bus standard used to regulate the connection and communication between computers and external devices.


Asynchronous serial interfaces such as EIA RS-232, RS-485, and RS-422 are used across multiple industries. 


Devices such as Remote Terminal Units (RTUs), Programmable Logic Controllers (PLCs), Point-Of-Sale terminals (POS), modems, and routers all use serial interfaces to communicate. 


Due to the limitations of the initial standard and subsequent revisions, serial interfaces have a large number of physical pinouts, so it is often incredibly challenging to get devices from different manufacturers to communicate over these interfaces. 


This white paper describes how to connect these devices to Cisco IoT routers. 


It also discusses various serial interface use cases, configuration, and troubleshooting.   


DTE and DCE ports in RS-232 RS-232 to RS-485 adapter


DTE and DCE ports in RS-232   


RS-232 to RS-485 adapter



The RS-485 standard does not have a connector specified. 


Most vendors provide screw connectors. 


Most Cisco IoT routers have native RJ45 RS-485 serial ports that allow connectivity to RS-485 devices. 


For routers such as the Cisco 1101 Industrial Integrated Services Router (IR1101) that do not have native RS-485 interfaces, RS-232 to RS-485 adapters can be used. 


Some examples of adapters are shown below. Apart from SEL-2886, other adapter models are provided for reference only and haven’t been tested in a Cisco lab.


RJ45 to DB9 adapterRJ45 to DB9 configurable adapterConfigurable RJ45 to DB9 adapter



RJ45 to DB9 adapter  


RJ45 to DB9 configurable adapter


Configurable RJ45 to DB9 adapter


RJ45 to DB9 adapter that comes in both DB9 male and female configurations and can be used to connect RJ45 ports on Cisco IoT routers to various serial devices.



RJ45 conductor terminalRJ45 male plug 8-conductor terminal


RJ45 conductor terminal   



RJ45 male plug 8-conductor terminal



RJ45 conductor terminal that can also be used to connect serial devices to Cisco router serial ports in cases where there is a need to terminate wires directly and even for troubleshooting physical layer connectivity.


DB9 breakout connector DB9 breakout Terminal


RJ45 to DB9 female adapterRJ45 to DB9 male adapter


DB9 breakout connector   


DB9 breakout Terminal


RJ45 to DB9 female adapter


RJ45 to DB9 male adapter


DB9 breakout connector that can also be used when there is a need to terminate wires directly and even for troubleshooting physical layer connectivity.



RJ45 DB9 straight adapterDB9 RJ45 blue Cisco console cable


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