The Indactic 21 protocol was developed in the eighties by the BBC
company (today ABB) in order to connect their RTUs and SCADA systems.
Indactic 21 encodes the data in a proprietary PCM code. It uses a
synchronization signal with a pulse of 1.5 length (i.e. 1.5 / baud
rate) to ensure unambiguous recognition of the telegram start. In practice baud
rates between 200 and 600 Baud are used. And although these baud rates are quite
low, the data volumes transmitted can be relatively large, as the encoding is not
burdened with unnecessary information.
There are several protocol dialects, so the actual function scope may vary.
Spontaneous transmission of information changes by a
substation without an explicit request or polling by the master is quite rare.
Additionally, the positive summary acknowledgment is applied, where the master
acknowledges all telegrams received in the meantime in regular specified
intervals in one go.
Indactic 21 Application Layer
RS232 / V.24 (PCM)
Supported Information Types
Single indication [1 Bit]
Double indication [2 bit]
Measured value [8 bit] unipolar
Measured value [8 bit]
Measured value [9 bit]
Measured value [11 bit]
Tap position BCD
Counter value dual [8 bit]
Counter value dual [16 bit]
Counter value dual [32 bit]
RS232 / V.24 (PCM) PtP
Transmission of pulse commands Spontaneous transmission of Indications, Measurands and Counters General Interrogation Grouped interrogation of Indications, Measurands or Counters
Station Address: 1..127 Information Type: as defined in protocol description Group Address: 0..127
Available Protocol Stacks
Indactic 21, Master
Universal protocol converter for highest degree of flexibility