CS-105 Auxiliary Bus

Introduction

The CS-105 Auxiliary Bus can be configured for different protocols including:

1. Cab Bus
2. XpressNet^TM

Best Practices

The Auxiliary Bus should be a continuous linear bus up to 1000 feet or 300 meters in length. Branches should be avoided. The CS-105 can provide up to 400mA of power for the Auxiliary Bus. Using a 12V supply, the CS-105 outputs about 10V and using a 15V supply, the CS-105 outputs about 13V. Additional supplemental power may need to be added depending on the length of the bus and the number of items plugged into it. Supplemental power should be in the 12V to 15V DC range, and distributed throughout the full length of the bus. Do not use termination. The CS-105 includes all the necessary termination internally.

Troubleshooting Tip

1. Check for bad cables.
2. Check for having enough power.
   1. The bus voltage should not drop below 9V at any point under maximum load (all devices plugged in and turned on).
3. Check for unnecessary termination.
   1. Do not use termination, the CS-105 includes all the necessary termination internally.
4. Check for proper configuration. The CS-105 must be configured for the expected protocol. The default from the factory is Cab Bus.
   1. From a TCS UWT throttle: Menu -> 8 Settings -> 2 DCC System Settings -> 2 System Settings -> 4 Aux Cabs -> 1 Mode
5. Ensure all the bus items have a unique ID.
   1. Valid Cab Bus IDs are 2 to 63
   2. Valid XpressNet IDs are 1 to 31

Known Issue

Summary

Some early CS-105 production units have a less than ideal value for one of the Auxiliary Bus components. The majority of users will likely never experience this issue. However, in some larger Auxiliary Bus networks, this can lead to unreliable operation on the Auxiliary Bus.

Affected CS-105 serial numbers include 09.00.99.03.00.00 to 09:00:99:03:01:5F. CS-105 systems with serial numbers outside this range are not affected. If you have a CS-105 in this range, and are experiencing Auxiliary Bus reliability issues, contact TCS support to discuss a resolution. TCS will go through some troubleshooting steps. If this known issue is identified as a likely cause, TCS may ask for the unit to be returned so that a small hardware update can be applied.

A multi-meter can be used in order to verify the CS-105 status. Power down the CS-105 and measure the resistance between the two center pins of the Auxiliary Bus connector (pins 3 and 4 of the 6-pin RJ-12 connector). It is usually easiest to measure this value at the closest panel connected to the CS-105. If the resistance value is less than 1K ohms, then the CS-105 has the original, less than ideal component value. A typical reading is in the 120 ohm to 200 ohm range, but anything <1K ohms indicates the hardware issue.

This issue does not affect any other interfaces on the CS-105. Only the Auxiliary Bus is affected.

Acknowledgements

TCS would like to acknowledge the Lehigh & Keystone Valley Model Railroad Museum located in Bethlehem, Pennsylvania USA in their efforts working with TCS to identify the root cause and validate the solution. This is an amazing museum quality model railroad and well worth a visit. Open house dates and times are posted on their website.

Technical Analysis

It is not necessary for users to understand the technical details of this issue. This explanation is provided for those users who are curious and have the necessary technical background, such as a degree or practical experience in electrical engineering or similar field.

The Auxiliary Bus uses a hardware standard commonly known as RS-485. Sometimes also referred to as TIA/EIA-485 within standards organizations. RS-485 uses a differential signal between two A and B signal lines to represent 1 and 0 states. This makes it relatively immune to certain types of environmental noise. A 1 state is represented by a positive differential signal, and a 0 state is represented by a negative differential signal. Only one device can transmit in the network at any given time.

When no devices are transmitting, the signal should return to an idle 1 state. Modern RS-485 hardware can interpret no differential voltage as an idle 1 state, however, older RS-485 hardware must still observe a positive differential signal, typically >200mV in magnitude. In order to achieve this, the CS-105 has a weak pull-up to +5V on the A line and a weak pull down to 0V on the B line. This is called "weak" or "fail-safe" biasing, and is required when there is any possibility of using older RS-485 hardware that relies on it. The CS-105 is designed to work with products that are more than 30 years old, and so the CS-105 uses weak biasing to ensure compatibility.

Many RS-485 networks implement termination as a resistor between the A and B lines. The primary purpose of this termination is to dampen the ringing energy that can reflect along the length of the RS-485 network. Termination can have a considerable dampening effect on weak biasing. This is the source of the issue in the CS-105.

The original termination value used in the CS-105 clamps the idle differential voltage to ~375mV under light load and ~300mV under full load (maximum number of devices plugged in). While this is still greater than the required >200mV, TCS has found that under some network conditions, the signal can be distorted out of spec. This includes:

1. Particularly long networks that also…
2. have less than ideal network wiring. Such as:
   1. use of non-impedance controlled cabling.
   2. use of flat or non-twisted pair cabling.
   3. close proximity to other high frequency signals such as DCC bus wiring.
   4. non-trivial branches.

Hysteresis can also play a factor in further reducing the idle differential voltage margin, and some RS-485 hardware can have hysteresis values of 50mV or more.

Model Railroads are often a less than ideal environment electrically, and such less than ideal networks are commonly found on large model railroads.

Based on real world testing, including in less than ideal electrical environments with less than ideal network wiring, TCS has determined that the two most commonly used protocols (Cab Bus and XpressNet) are reliable when used with much less termination than what was originally used in the CS-105. TCS has chosen a weaker termination value going forward that provides adequate dampening of reflections without unnecessarily clamping the idle differential voltage.

One of the determinate factors for necessary termination strength is the baud rate of the protocol. Both Cab Bus and XpressNet protocol have relatively low baud rates by RS-485 standards, and therefore do not require strong termination.