Data transmission method

The invention relates to a method for the asynchronous serial data transmission between a transmitter and a receiver over a radio transmission link, whereby a synchronization data frame and a carrier recognition frame is arranged in front of the useful data frame. The data frames are each enframed at the beginning and end by a start bit and a stop bit at the beginning and end. The coding of all data frames results such that, in addition to the start and stop bits, an equal number of zero and one bits exists.

The present invention relates to a method for asynchronous serial data transmission as claimed in the preamble of patent claim1.

In this context, it is known for a carrier identification and synchronization to be sent in advance of user data transmitted between a transmitter and a receiver. The carrier identification signals to the receiver that a message is being sent, and the synchronization is used to synchronize to the start of the user data, that is to say to the start bit preceding this user data.

The simplest form of transmitter and receiver synchronization is used for asynchronous transmission. The transmitter clock and the receiver clock in this case need be only at approximately the same frequency. The transmitted data words have a fixed format, and are also provided with a start bit and a stop bit. The receiver clock is synchronized to the negative edge of the start bit, and the other bits are sampled in the center of the bit time period.

Asynchronous data transmission can also be carried out with low-cost standard modules, such as those marketed by various manufacturers in the form of UARTs (Universal Asynchronous Receiver Transmitter). These have internal clocks whose time constancy is not subject to any particularly stringent requirements, since the clock is in each case resynchronized at the start of a character, that is to say in general after the transmission of 10. Thus, all that is necessary is for the stop bit still to be sampled during the bit period.

For data transmission, it is also important for the transmitting/receiving electronics to have a constant operating point, and for the operating point not to be shifted, for example, due to lengthy sequences of zeros or ones in the character coding. This could be ensured by using coding formats without any direct-current element, but in some cases these are too complex and cannot be evaluated by UARTs.

The object of the present invention is thus to specify a data transmission method which ensures that the transmitting/receiving electronics have a constant operating point, while using a simple coding format.

This object is achieved by the description features of patent claim1. Further advantageous refinements of the method according to the invention can be found in the dependent claims.

As shown inFIG. 1, the bit stream used for the transmission method is composed, in a known manner, of a carrier identification, synchronization and the user data. Each part of this bit stream is composed, as shown inFIG. 2, of a character with a width of 10 bits, with the first bit being a start bit and the last bit being a stop bit. The start bit is represented by a “0”, and the stop bit is represented by a “1”. In between, there are eight information bits for the character.

As shown inFIG. 3, the carrier identification is coded based on the hexadecimal code55, that is to say ones and zeros alternate with one another. The bit rate is, for example, 9 600 baud, which corresponds to a frequency of 4.8 kHz. The internal clock in the UART also operates at this bit rate or frequency for sampling the character bits.

As shown inFIG. 4, the synchronization character is coded using the hexadecimal code33, that is to say two ones are in each case followed by two zeros etc. This code is symmetrical, as far as the number of ones and zeros is concerned, so that the direct-current level and the operating point of the receiving electronics do not change, on average, and with the frequency of 2.4 kHz still being sufficiently high for radio transmission. Furthermore, the synchronization character is chosen such that its bit pattern does not occur in the user data.

As shown inFIG. 5, the user data are coded using Manchester coding (bi-phase format), in which both the zeros and the ones are coded by pulses, although these occur at different times within a bit frame. For example, a one is represented by a square-wave pulse with half the step duration which is located in the first half of the time period available for representation of a bit. The same pulse is used to represent the zero, but in this case is located in the second half of the time period. The Manchester code thus ensures the transmission of clock information as well in the “0” and “1” sequences, and it is possible to distinguish between the transmission of a series of zeros and no transmission. Although the signal does have a direct-current component, its level is the same, however, on average.

Since each zero or one is coded by means of a pulse having half the step width of the carrier frequency of 4.8 kHz, a byte with 8 bits cannot be coded in one character; in fact, two characters each preceded by a start bit and followed by a stop bit are required to transmit one byte, which represents a special feature of the present invention.

For example, in the example illustrated inFIG. 5, the user data are coded in the hexadecimal code21.