S88 - Analyzing the transmission


    The S88 bus is established as cheap feedback bus model railway layout. The principle is simple: the S88 bus is just a serial shift register with a parallel load input.

    Additional modules are added to this bus by simple cascading, thus creating a long shift register containing all bits in one long chain. This advantage (simple topology) comes together with some drawbacks: The actual address is defined by the position in the chain (unchangable), the data transmission is made completely unprotected, ie there is no parity, no checksum nor CRC.

    Data Transfer:

    The LOAD line goes to 1, then a shift clock is applied. All registers in the chain take over the information at their parallel inputs. The next step is a RESET pulse (also active high), this clears the parallel input upstream latches. These latches are ready again to capture new feedback events. Thus even very short signals are latched until the next query.
    Now, the shift occurs (with LOAD = 0). Because each S88 data output of a module is connected to data input of the next module, all information stored in the shift register is shifted step-by-step towards the command station.

    Unfortunately, this relatively trivial technique is often described as unreliable. This is cause by the electric inadequate realisation. Unshielded ribbon cables are used and these cables are operated without line termination and with weak drivers (high impedances). No wonder, that sometimes crosstalk and reflections are observed.


    To improve the electrical side of this transmission, following measures should be made: termination, separation and shielded cables..
    • Termination depends on the output power of the used command station. In OpenDCC TTL bus driver chips are used, which can supply up to 24mA. Here it is possible to achieve a significant reduction of clock overshot by terminating with 220 ohms against 5V at the end (!) of the line.
    • Crosstalk is easily reduced by separation. Do not route s88 bus lines along track signals or dimmed power supplies. Additionally, the track wiring should always be done with twisted wires, this reduces emmision fields.
    • Besides mechanical separation a proper electrical isolation must be done. No currents from the layout are allowed to run along the feedback bus (intentionally or not). Please have a lock at the wiring notes given with the booster.
    • A good protection against crosstalk is acheived with shielded cables, which are available very cheap as a patch cables (CAT5) for computers. Inside a CAT5 cable there are 4 pairs, each pair is twisted. Using the S88-N pin allocation, both shielding and twisting is used to acheive optimal conditions.