essing and unit identification. A message then commences with a 16 bit synchronising word as Data transmission 46/11 PREAMBLE SYNC WORD ADDRESS CODE WORD OPTIONAL DATA CODE WORDS BIT NO. NO. of BITS Figure 46.10 MPT1317 digital code format for data over CHECK BITS shown in Figure 46.11, to enable the decoder to establish code word forming. Messages are transmitted in 64 bit code words, each message may occupy one or more code words as determined by the message Figure 46.13 MPT1317 data code word structure length. Each code word contains 48 information bits and 16 check bits which are used for error detection and data code words. There are two types of code word, address code words and data code words. 46.8.4 Encoding and error checking The 16 check bits are calculated in three stages. Firstly, 15 check BIT NO. bits are appended to the 48 information bits by encoding them in a (63,48) cyclic code. For encoding, the information bits 1 to 48 may BIT VALUE be considered to be the coefficients of a polynomial having terms from x62 down to x15. This polynomial is divided by modulo 2 by the generating polynomial as in Equation 46.3. Figure 46.11 MPT1317 synchronisation frame 15 14 13 1 (46.3) 46.8.2 Address code word The 15 check bits, code word bits 49 to 63, correspond to the coefficients of the terms from to x° in the remainder polyno- mial found at the completion of the division. The (63,48) cyclic code The first code word of every message is an address code word as has a minimum distance of 5 and SO guarantees detection of up to 4 shown in Figure 46.12. Bit 1 is always binary 1 to distinguish the bit errors in one code word. word from a data code word, bits 2 to 8 inclusive specify the user's Secondly, the final check bit of the (63,48) cyclic code (code word identity, bits 9 to 48 inclusive may be assigned to represent ad- bit 63) is inverted to protect against misframing in the decoder. dresses and data, and bits 49 to 64 inclusive are check bits. Within Thirdly, one bit is appended to the 63 bit block to provide