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RS Feedback

The RS Feedback Bus was developed by Lenz and adopted by the NMRA.

Feedback is generally used to control trains, point switches and/or signals in a model railroad.

Feedback allows a computer to keep track of where trains are on the track, and for that, it needs to know which sections are occupied, this is done by occupancy detectors.

The Occupancy detectors provide a Feedback signal which is  connected to a feedback module that informs the computer/command station.

The signal of the occupancy detector arrives at one of the inputs of the RS feedback modules (it is connected to ground) and this change gets reported via the RS Feedback bus to the command station or computer.

It is possible to use up to 128 RS feedback modules with 8 inputs (each one having a different address).

In modular scale models the RS Feedback Bus has greater advantages over the S88 feedback Bus (because having a fixed address it doesn’t depend on the order in which they are connected)

The RS Feedback Bus only transmits changes and does not continuously transmit the states of all inputs (very noisy).

The RS Feedback use 2 wires as apposed to the 6 wires used by the S88 Bus



For switches signal or other feedback, there are various bus solutions for cable saving. Is known of the S88-feedback or LocoNet, Selectrix® and RS bus. The RS-feedback is a proprietary solution of Lenz for their headquarters LZ100 / LZV100. While the other buses are fairly well documented, the RS bus is not available in virtually undescribed. Inquiries revealed no essential information. An attempt is made to describe the essential properties.

Advantages over the bus S88 are located in the transmission of information by current flow and not by voltage level as at S88. This results in a significantly lower Einkopplungswahrscheinlichkeit. In addition, the data (as opposed to S88) protected by a parity.

Not the subject of consideration is the actual busy / feedback as well as the internal circuitry and programming of the blocks. Conceived as a result of the measurements block a Contact Indicator / track occupancy detector was practically built and is possibly available.

The RS bus is a pure signaling bus that operates in mono master mode. The LZ100 or similar centers are operating as master. News of the RM only after the start and changes, the new state will each appear.

The panel sends 130 pulses, each feedback module (RM) hears the message with. This results in a pulse train of 130 pulses with approximately 109us High and Low around 93us pulse and a pause of about 7 ms. (see Figure 3) If a current message is ready in a RM, he or she shall after the pulse corresponding to its address on the bus. The panel waits until the end of the message with the next pulse. Each response of a block consists of 4 bits (1 nibble). If a repeater at its inputs change from two nibbles, these messages are delivered in 2 cycles. Blocks with different address to send its message during a cycle. There are actually so there are always two quad-RM, which share a common address and do not send the same time.
physical realization

All of the tested building blocks that are

LR101 Fa. Lenz
RS8 Fa. Littfinski
GBM16 Fa. Blücher
LS100 Fa. Lenz (decoder with RM)

cooperate with the LZ100 / LZV100 the Fa. Lenz, has defined the RS bus.

Figure 1: Principle feedbacks

Participants circuit or bus interface is at these companies iw equal solved and inspired by the LR101. Little things, like another optocoupler (OK) or the use of a current regulating diode instead of a constant current source with LM334 are insignificant. There are two power sources, one with and one with 3mA approximately 20mA. The 3mA current source is used for receiving, the 20mA current source is switchable, and is used for transmission. In series with 3mA current source is still a zener diode 5.6V. Due to the duty cycle of about 50% results in a quiescent current of about 1.6 mA / RM.

Figure 2: Principle Central

The Center contains Taktsende- and evaluation circuit for the RS bus. At the terminal R + 12V provided. Port S via a transistor T2 and resistors to ground. In parallel, there is a switching transistor T1 directly to ground, the switches 130 transmit pulses.

All RM-blocks are parallel to R and S. When ON, the clock transmitter S is pulled to ground, it incorporated them about 3mA / RM on T1. When T1 is switched off, the current through T2 and the voltage divider R1 must R2 flow. T2 (in base) realized this together with R5, R6, a threshold that makes a significant current to flow only at voltages above 6V to south. This voltage is not reached by the 3mA current source due to the Zener diode connected downstream. Therefore, the voltage across R2 is not sufficient for carrying controlling T3, which produces the receive signal for the processor in the central office.

If a RM sends a message, it clocks the 18mA current source, which operates without series Zener diode. This current produces a voltage drop, through the controls T3, and therefore generates a low pulse for the feedback processor with activated T2 in R2. During the response time, no new transmit pulse is generated by the control panel. Only after the end of the 9-bit answer the next clock pulse is sent.

By realized for all RM circuit with optocouplers galvanic isolation of the RM is secured to each other and from the central office.

This should not be repealed by a common power supply. Since the different types of RM generate different reference potentials, the LZ again another, blocks can be destroyed.
Start behavior / application

The LZ100 is after switching from a pulse about 88ms with subsequent pause about 562ms. The same applies after disturbances. During this time, the RM can reset their settings and register after the start of normal pulse telegrams.

The tested blocks

the LR101 (Lenz)
the RS8 (Littfinski)
the GBM16 (Blücher

Send your registration immediately after the start. However, the application of a RM is also possible later at random times.

A block logs on by sending a valid lower and upper nibble in 2 successive pulse telegrams. Sending the nibbles in non-consecutive cycles or eg 2 Lower or upper nibbles not lead to the application. A cancellation does not exist. Every message that arrives at time of a notified RM’s is associated with the address. Sends an undeclared RM a message, the pulse sequence of the LZ100 is not stopped, as otherwise notified in RM.
Coding / protocol

The message is sent with a’Feedback 4800bit / s. It consists of 8 bits of user data:

Start bit (0)
Parity (even)
2 TT bits (with repeater: 10)
Nibblebit (0 = lower nibble, 1 = upper nibble)
4 data bits D3, D2, D1, D0
Stop condition (RM passive again)

This is for D3 … D0 input E1 E4 or E5 … … E8. A 1 corresponds contact is closed, so really low level! Bit 5 and 6, the TT bit. They contain the identifier of the feedback module. They are in a different context in the bilge Doc .:

00 – Switching receiver without feedback
01 – Switching Receiver with feedback
10 – Position indicator
11 – reserved for future use

It is not to understand here is why such exotic encoding was chosen. The TT bits represent no relevant information is only to suggest that, for historical reasons (nibble information) is because no ancient mC (8 bits + parity) -. Could spend telegrams. In addition, the demands on the accuracy of the clock frequency in asynchronous short bit sequences are lower. As a result, so it is said that ultimately only 4 bits can be issued per message. A message about this nibble says absolutely nothing about the other nibble, the message is completely separate, but always in a different cycle. These are then encoded as follows (in RM!):

P 1 (t), 0 (t), nibble, D3, D2, D1, D0

P is sent first, D0 last, in contrast to conventional UART’s. Within a nibble, it is possible to send multiple information, eg Input 1 and 3 active (0 ..) resulting 001,000,101, but not information on various nibbles. This must be distributed as 2 messages to 2 cycles.
Figure 3: RS-BUS: Pulse repetition

As a result, so it is said:

different detectors can be received in one cycle
lower / upper nibble of a detector require 2 cycles

Sorry, thus eliminating the possibility of implementing small, arranged distributed 4p RM modules if you want to show the address space without gaps. This should always be with the RM that monitors the other nibble at the same address, synchronize. Only when dispensing with 4 inputs, 4 beded structure is conceivable, but not particularly useful, since the difference from the 8-RM is only in a few passive components that cost a few cents.

The graph above shows the pulse sequence RS bus with no response (130 pulses 202μs + Pause 7ms), below the Pusfolge (detail) with response of the RM.
Messages (examples)

Messages are coded as follows:

St, Pe, T1, T0, N, D3, D2, D1, D0

St – Start bit (Low)
Pe – parity (even)
T1 – T1, high fixed at RM (feedback module)
T0 – T0, laid low at RM
N – Nibble (0 for bottom, 1 for upper nibble)
D3 – E4 or E8 for
D2 – for E3 and E7
D1 – for E2 or E6
D0 – for E1 or E5

with input closed (= low) corresponds to a 1. A message describes only a nibble of simultaneous changes in both Nibbles two messages are generated.

Some examples:
No, St Pe 1 0 N D3 D2 D1 D0 Meaning
1 0 0 1 0 0 0 0 0 0 E1, E2, E3, E4 passive
2 0 1 1 0 0 0 0 0 1 E1 active, passive E2..E4
3 0 1 1 0 0 0 0 1 0 E2 active, E1, E3, E4 passive
4 0 0 1 0 0 0 1 0 1 u E1. E3 active E2 u. E4 passive
5 0 0 1 0 1 0 0 0 1 E5 active, E6, E7, E8 passive
6 0 0 1 0 1 1 0 0 0 E8 active, E5, E6, E7 passive
7 0 1 1 0 1 0 0 0 0 E8 passive again
8.1 0 1 1 0 0 0 0 0 1 2 messages: E1 active and …
8.2 0 0 1 0 0 0 0 1 0 … E6 active, passive residual
9 0 0 0 1 1 0 0 1 0 LS100A1- (E6) are active, passive residual

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