The protocol is quite simple: every minute the full time and date is transmitted in 59 bit (from 0 to 58). At the beginning of every second a bit is transmitted by reducing to 25% (-6dB) the 77.5kHz carrier (amplitude modulation) for 100ms (logical 0) or for 200ms (logical 1). The 60th bit is not emitted and this is used as a marker for the beginning of the next code.
| Bit | Name | Description | Comment |
| 0..14 | - | Reserved | |
| 15 | R | Antenna bit | 1 when secondary antenna is used |
| 16 | A1 | Delta DST bit | 1 when DST change occurs in next hour |
| 17 | Z1 | Time zone bit 1 | 1 when DST is set |
| 18 | Z2 | Time zone bit 2 | usually the opposite of Z1 |
| 19 | A2 | Commutation bit | 1 when a 61 seconds minutes is inserted |
| 20 | S | Start bit | always 1 |
| 21..27 | M | Minutes | 7 bit (BCD, LSB first, 00..59) |
| 28 | P1 | First parity bit | Even parity for all transmitted bits |
| 29..34 | H | Hours | 6 bit (BCD, LSB first, 00..23) |
| 35 | P2 | Second parity bit | Even parity for all transmitted bits |
| 36..41 | DM | Day of month | 6 bit (BCD, LSB first, 01..31) |
| 42..44 | DW | Day of week | 3 bit (BCD, LSB first, 1=monday..7=sunday) |
| 45..49 | MN | Month | 5 bit (BCD, LSB first, 01..12) |
| 50..57 | Y | Year | 8 bit (BCD, LSB first, 00..99) |
| 58 | P3 | Third parity bit | Even parity for all transmitted bits |
Three times per hour (during the 19th, the 39th and the 59th minute) the sender ID (DCF77) is emitted in Morse code by a 25% amplitude modulation of 250Hz between second marks (without suspending time signals).
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