RS-232C protocols with electrical properties

In the electrical industry, we should be familiar with RS-232C protocols. As the protocol standard for voltage signal control, it has been applied in many fields. Next, let's take a brief look at this protocol. First, let's explain the specific meaning of this Protocol.

RS-232-C

The RS-232C protocol is the EIA-RS-232C protocol, in which EIA (Electronic Industry Association) represents the American Association of electronics Industry, RS (recommeded standard) represents the Recommendation standard, 232 is the identification number, C Represents the latest version of RS232 (1969), which was previously RS232B and RS232A. It specifies the connection cable and Mechanical, Electrical Properties, signal functions and transmission process ｡

Commonly used physical standards are RS-232-C, RS-422-A, RS-423A, RS-485. Here only introduces the RS-232-C (232, RS232). For example, the current on the ibm pc COM1, COM2 interface, is the RS-232C interface ｡

1. Electrical Properties

EIA-RS-232C protocol for electrical properties, logical electrical and various signal line functions are defined ｡

On TxD and RxD: logic 1 (MARK) =-3V ~ -15 V

Logic 0 (SPACE) = + 3 ~ + 15 V

On the control line such as RTS, CTS, DSR, DTR, and DCD:

Effective Signal (ON, positive voltage) = + 3V ~ + 15 V

Invalid signal (disconnected, OFF, negative voltage) =-3V ~ -15 V

The above describes the definition of the logical level of the RS-323C standard ｡

For data (Information Code ):

The level of the logical "1" (PASS) is lower than-3 V, the level of the logical "0" (empty) is + 3 V; for the control signal; ON) that is, the valid signal level is higher than + 3 V, and the disconnected state (OFF) means that the invalid signal level is lower than-3 V, that is, when the absolute value of the transmission level is greater than 3 V, the circuit can be effectively checked out, ranging from-3 ~ The voltage between + 3 V is meaningless, and the voltage lower than-15 V or higher than + 15 V is also considered meaningless. Therefore, during actual operation, the voltage at ± (3 ~ Between 15) V ｡

EIA-RS-232C protocol and TTL Conversion:

The RS-232C protocol uses positive and negative voltage to represent the logical state, which is different from the TTL to express the logical state in high and low levels. Therefore, in order to be able to connect with the computer interface or terminal TTL device, the EIA-RS-232C protocol and TTL circuit must be converted between the power level and the logical relationship, it can also be used as an integrated circuit chip. At present, it is widely used as an integrated circuit converter. For example, the MC1488 and SN75150 chips can convert TTL levels to EIA levels, while MC1489 and SN75154 can convert EIA level to TTL level ｡

2. Mechanical Characteristics of connectors:

Connector: Due to RS-232C protocols and undefined physical properties of the connector, there are various types of connectors for DB-25, DB-15, and DB-9, the pin definitions are also different. The following describes two connectors ｡

(1) DB-25: PC and XT machine using DB-25 connector. DB-25 connector defines 25 signal lines, divided into four groups:

① Nine voltage signals (including signal ground SG) of asynchronous communication are 2, 3, 4, 5, 6, 7, 8, 20, 22

② 20mA current ring signal 9)

③ 6 blank)

④ One protected area (PE), used as the grounding end of the equipment (foot)

Note: The 20mA current ring signal is only provided by ibm pc and ibm pc/XT machines. It is not supported by AT machines or later ｡

DB-9 connectors (2)

AT machine and later, do not support 20mA current ring interface, use DB-9 connector, serves as a connector that provides both a multi-functional I/O card or a COM1 and COM2 serial interfaces on the motherboard. It provides only nine signals for asynchronous communication. DB-25 Connector pins are allocated with DB-25 pins. it is completely different. Therefore, dedicated cables must be used if connected to a DCE device with a DB-25 connector ｡

Cable Length: when the communication rate is less than 20 kb/s, the maximum physical distance directly connected by the RS-232C protocol is 15 MB (50 feet )｡

Maximum direct transmission distance Description: RS-232C protocol, if no MODEM, in the case of code distortion less than 4%, the maximum transmission distance between DTE and DCE is 15 m (50 feet) it can be seen that the maximum distance is given on the premise that the code source distortion is less than 4%. To ensure that the code source distortion is less than 4%, the interface standards are specified in electrical characteristics, the load capacity of the drive should be less than 2500pF ｡