RS232 Signal Levels:

The voltage levels are one of the major specifications. For RS232 data signals, a voltage amid- 3V and - 25V represents logic 1. A voltage amid + 3V and + 25V represents the logic 0. Control signals are in the "ON" state if their voltage is amid + 3V and + 25V and "OFF" if they are negative, that means between - 3V and - 25V. The data is serially sent on RS232, each bit is sent one after the next since there is only one data line in each direction. This mode of data transmission also needs that the receiver knows when the actual data bits are arriving so that this can synchronize itself to the incoming data. To attain this logic 0 is sent as a start bit for the synchronization. It is followed by the data itself and there are seven or eight bits normally. Obviously the receiver needs to know how many data bits to expect, and there are frequently small dual in line switches either on the back of the equipment or inside it to set this information. Normally Data on RS232 is sent by using ASCII (American Standard Code for Information Interchange). Although other codes can be utilized equally well. After the data itself a parity bit is sent. Again it needs setting because it is optional and it can be even or odd parity. It is used to check the correctness of the received data and this can indicate whether the data has an odd or even number of logic ones. Unlike several systems these days there is no facility for error correction. At last a stop bit is sent. It is normally one bit long and is utilized to signify the end of a particular byte. Sometimes two stop bits are needed and again this is an option that can frequently be set on the equipment. Normally RS232 data transmission is asynchronous. But transmit and receive speeds must obviously be the same. A certain degree of tolerance is permitted. Once the start bit is sent the receiver will sample the centre of each bit to see the level. Within every data word the synchronization should not differ by more than half a bit length or else the incorrect data will be seen. Luckily this is very easy to obtain with today's accurate bit or baud rate generators.