Copyright Kersten Tams (http://www.tams-online.de) and Wolfgang Kufer (http://www.opendcc.de).

Current situation 01.01.2010

 

S88 over RJ45

Current situation :

    The S88 bus is established as an inexpensive feedback bus for model layouts.
    Technically speaking it is not even a data bus, but one big shift register. The feedback decoder translates its contacts into bits and bytes. When the status of the contacts is read, all the bits are pushed one by one to the central unit. There is no error correction or any other technique to suppress errors

    Its simple nature make the S88 bus vulnerable for disturbance during the transmission. A switch over to cheap network cables will greatly reduce the errors while reading the feedback modules.

    The advantages of CAT-5 cabling and RJ-45 plug connectors are also recognised by other designers. Unfortunately everybody used different pin assignments. Mid 2007 a standard was developed to make the new generation of S88 decoders compatible: s88-N. Future developments should use this standard s88-N . There are already several manufacturers who have adopted the new standard. Here are links to those manufacturers.

Pin allocation

    s88-N-Logo s88-N: Standardization of s88 on network cable (CAT-5, RJ45)
    Pin S88 Name Description Pin RJ45 colour
    1 DATA data to read 2 gn
    2 GND Return (Ground) for signals and power supply 3 wt/or
    2 GND 5 wt/bl
    3 CLOCK clock signal for shifting data 4 bl
    4 PS (LOAD) load request for slave devices 6 or
    5 RESET clear the slave devices 7 wt/br
    6 +5V/+12V power supply to feedback modules 1 wt/gn
    - RAILDATA *) track signal (e.g. DCC or MM) *) 8 br
    - SHIELD cable shielding, not connected - -


    (*) RAILDATA should be routed through even when not used.)
    (Click to logo to get a better image)

     

Guidelines

    The use of the logo, of the name s88-N and the cable pinning is free of charge. However, to ensure the compatibility between different devices, following rules must be observed:
  • pin-out according to the above table
  • compliance with the timing requirements and publication of the implemented timing.
  • clear indication whether 5V - 12V operation is supported.
  • clear indication whether RAILDATA (with or without feedback) is supported. The logo and name is copyrighted by Kersten Tams and Wolfgang Kufer. Any usage which extends the signing of modules and components compatible to s88-N requires a written approval of the copyright owners. Modules which are not conform to the above specification and condition are not not allowed to use the logo.
    Every user of the logo and the name s88-N must announce this to the copyright owners. (with mail).
    Every user has to reference to the source of the logo (could be a link to either http://www.opendcc.de, http://www.digital-bahn.de or http://www.tams-online.de).

Overview and comparison of past assignments s88 to RJ45

    Overview of past assignments
      Vendor / Standard 
    Pin RJ45digital-bahn.deopendcc.derailway-lauf.deiek.des88-N
    1 +12V GND DATA DATA +12V/+5V
    2 DATA CLK GND DATA DATA
    3 GND GND GND GND GND
    4 CLK PS CLK CLK CLK
    5 GND GND PS PS GND
    6 PS RESET RESET RESET PS
    7 +12V +5V +5V +5V RESET
    8 RAILDATA DATA +5V +5V RAILDATA

    Following comments on these pin-outs:
  • The assignments of railway lauf.de and iek.de are electrically unfavorable, because the sensitive lines are not paired with static lines. Both assignment were obviously driven by the wish of having a simple pcb layout. To make things worse, the assignment of IEK does not strengthen the GND.
  • A possible higher supply voltage of 12V and an appropriate shift of the threshold for the signals offers more margin, so that disturbances cannot affect the signal lines. Therefore also the 12V supply was allowed as an alternative to the 5V supply. The user must pay attention that only if all attached modules are tolerant to the higher supply than 12V may be turned on by the command station or by a s88 booster. Else the 12V supply may cause damage to the decoder.
  • In the assignment of digital bahn.de the RESET line was omitted, which may cause compatibility problems. The RESET line is contained in s88-N.
  • The optimal GND assignment of opendcc was weakened in favour of RAILDATA (i.e. the track signal).
  • To avoid uncontrolled GND-loops or loss of secure isolation, the cable shield should be left open.
  • To avoid the cable colour for s88-N patch cables should be blue.