CMRI Notes


This section is not a DCC specific topic but about the popular CMRI bus which is the oldest of the class of "Layout Control Bus" (LCB) system used for accessory control. The CMRI bus is PC based and is intended to control everything on a given layout OTHER than the running of trains. This serial bus allows anything from a simple local control panel to a full CTC dispatching panel to monitor turnout positions and track block occupancy status to control motors and implement signal logic which in turn drives signals.

The CMRI bus is possibly the oldest (1985) and currently has the widest adoption as a LCB system. It predates DCC by 10+ years.

The CMRI bus is not the official NMRA's LCB bus standard. However, the NMRA does recognize contribution the CMRI bus standard has made to the hobby by allowing the CMRI bus to be listed as an alternative LCB standard on the NMRA Standards webpage. This site will become the official depository of official CMRI bus standards documents to allow everyone to benefit from it's development for both current and future users.


This webpage exist to discuss some hardware technical aspects of the CMRI bus that seems to be a ongoing source of confusion to many people. The confusion can be directly tied to the CMRI bus unique design. I have done this in a FAQ format.

1) What is the CMRI Bus.

The "CMRI bus" is hybrid of two different hardware bus communication standards, RS422 and RS485, to allows a common RS232 serial port on a older PC's to be extended to communicate over very long distances to multiple devices reading and writing data to each to control a Model Railroad.

2) What is a RS232 serial port?

RS232 is a digital communication standard that is a combination of both Hardware and Software that work together to allow two way communication.

Hardware: RS232 serial port communicates digital data at high speeds using just a few wires to given digital device. At a minimum, RS232 allows two digital devices to communicate to each other, back and forth or two way duplex communication, up to distances as far away as 15 meters (50Ft) using common cables. At a minimum, it supports both types of duplex communication using 2 wires wires and a ground wire. The standard included the physical definition of the connectors used.

Software : RS232 defines Baud Rate (speed rates, bits per second [bps]) of communication and the protocol or format in which data is sent.

3) What are the two types of duplex communication used with serial ports?

There are two type of two way duplex communication. Full Duplex and Half Duplex

Half Duplex: Two way communication but not at the same time. Both devices must take turns talking waiting for the other device to finish before it talks.

Full Duplex: Two way communication that can take place at the same time.

4) What is RS422?

RS422 accomplishes two things

RS422 is a hardware only standard that was created to allow a given communication system standard, such as RS232, to s

a) Send digital information up to 10Million bps (10Mbps)over short distances (12-15 meters) down to much slower data rates of 90 thousand bps (90Kps) over very ong distance just over 1000meters. How far and how fast depends on the cable characteristics.

b) Address the voltage differences between two different grounds separated by long distances. The is call common mode voltage.

using two different busses.contains a single RS422 bus (OUT/TX) plus a single RS485 bus (IN/RX) using only RS485 bus drivers chips.