Monitoring of trains is made possible by the presence of sensors located throughout the network and the establishment of magnets under the locomotives. Receptivity will be evidenced by the temporary closure of a contact, caused by the passage of a locomotive. This signal is saved by a S88 decoder. Each decoder can handle 16 receptivities. To manage a large number of receptivity, several decoders are necessary. They are connected between them and the head of this chain is connected to the box "INTERFACE". A network can have up to thirty S88 decoders.


The computer will request the reading of receptivity by sending a read command to the box "INTERFACE". This reading may be:
- Global or selective (All decoders or decoder in particular).
- With erasure or maintenance of receptivity recorded. Playback order will include two information encoded on each byte:
- The instruction code (maintenance or obliteration of receptivity).
- The reference to decoder (s) concerned by reading. They are transmitted in order, the instruction code in the lead, followed by the reference.

Instruction code
It can take two values:
- 192 Transmission and deletion of receptivities activated.
- 128 Transmission and maintenance of receptivity activated.


Value transmitted may belong to two areas: 129-159 or 193-223. This value depends on the number of decoders S88 "that includes the network and the type of reading done: global or selective.

Global reference

The reference is defined by adding the number of decoders 128 of the network. It can vary from 129 to 159.
- Network with a decoder: 129
- Network with 2 decoders: 130
- Network with 30 decoders: 159

Selective reading

The reference is defined by adding 192 to the row decoder concerned by reading. It can vary from 193-223.
- Reading 1 decoder: 193
- Reading the second decoder: 194
- Reading the 30th decoder: 223


Receptivity will be activated by a so 1 Each S88 decoder includes a silkscreen
- 1-8 on one side
- 9-16 across
It allows identify each of the 16 that are associated with receptivity. For each S88 decoder, there will be transmission of two bytes containing the first marked receptivity 1-8, while the latter marked receptivity 9-16, The receptivity 1 and 9 are the bits of two bytes transmitted, the receptivity 8 and 16 being the least significant bits, during a reading on all decoders, the computer receives an even number of bytes. The transmission of receptivity is the order of chaining starting box "INTERFACE" ie the first two bytes received match the state of receptivity associated with the "decoder S88" located closer to the box "INTERFACE". On the digital channel, the nomenclature used to reference each receptivity can easily identify its position among the different bytes received.