CV 15 and 16 - “Locking” A Decoder - Revised 2023-05-09

Why do these CVs even exist!?!? Well, there are two necessary, relevant reasons for them. First, assume you have multiple decoders in the same locomotive or rolling stock car (remember Cody Grivno’s runaway boxcar?) and they all have the same DCC address, one for the motor, one for lights, etc? In this case, if you change a CV in one, you guessed it; the CV in ALL of them gets changed.

The second reason is to prevent electrical shorts from scrambling a decoder. This has happened to the author more than once and from discussions on the forums, many other individuals.

A good analogy to get a basic understanding of the relationship between these two CVs is to think of CV 16 as a lock and CV 15 as the key to lock/unlock it. Programming CV 15 to the same value as what is in CV 16 allows the decoder to me programmed as you see fit. If the values are different then the decoder is “locked”. There are some minor differences between decoder manufacturers of what can be modified (and, in the case of Soundtraxx, what extras need to be done to enable locking) and we will discuss what those differences are below.

NMRA Standard Explanation

Located starting at line #150 here:

Configuration Variables 15, 16: Decoder Lock The Decoder Lock is used to change CVs in only one of several decoders with the same short address (CV1) or long address (CV17 and CV18) that are installed in the same locomotive (Author’s addition: or motorized rolling stock with lights, etc.). Assign a number to CV16 in each decoder (i.e., 1 to motor decoder, 2 to sound decoder, 3 or higher to other decoders) before the decoders are installed in the locomotive (author’s emphasis). To change a value in another CV of one of the installed decoders, first write the number 1 (motor), 2 (sound), or 3 or higher (other) into CV15, then send the new value to the CV to be changed. The decoders will compare CV15 to CV16 and, if the values are equal, the CV to be changed will be changed. If the values in CV15 and CV16 are different, the update will be ignored.

Note: Although not specifically stated, legal values for CV16 are limited to 0 to 7. 

The key takeaway here is to decide on a numbering scheme for the decoders in your multiple decoder equipped entities and to document what you have them programmed to. Without that you will be lost, confused and frustrated in no time.

Now let’s talk about the decoder manufacturer’s differences and similarities.

Digitrax

Digitrax follows the NMRA standard. They discuss the CVs in their Decoder Manual V2 here starting on page 66.

BLI and TCS

Both of these manufacturers follow the NMRA standard per their manuals. BLI  manuals are located here .

BLI also states

“Please note once the decoder is locked, no CV’s other than CV1 or CV15 may be read or changed.”

The TCS Comprehensive Programming Guide can be downloaded here. 

Soundtraxx

The NMRA standard is followed by Soundtraxx with one addition summarized by a note attached to both CV 15 and 16 in their Technical Reference manuals here.

Note: Bit 0 (CV Lock Enable) of CV 30 (Error Information) must be set to 1 in order for the lock feature in CVs 15 and 16 to be used

ESU

V5
The version 5 ESU manual states that they follow the NMRA standard. There is no mention of differences between the V5 DCC and V5 European decoders nor is there a discussion of how to implement locking so the author tested one of his v5 decoders (this was not a thorough test). Setting CV 16 to a value of 7 and CV 15 to a value other than 7 prevented any changes to other CVs. As soon as CV 15 was set to 7, other CVs could be changed. The author has no multi-decoder pieces to test.

V4
The version 4 (unofficial) manual here makes no mention of CV15 or CV16 specifically so the author cannot say anything about these. (the author has no V4 decoders, DCC or European). Also, there may be differences between the decoders designed for Europe and their counterpart DCC decoders designed for North America and Australia. If anyone tests this or knows for sure please leave a comment and I’ll update this post.

Additional Specific Decoder Findings - thanks to Mark Granville

Decoders Without Locking Features

• Atlas Dual Mode version4 
• Digitrax DH150, DH165, DN140. 
• This is consistent with the instructions & manuals not listing CVs 15 & 16 
• Interestingly, you can read and write these CVs in the DH165 but they have no effect
• ESU LokSound Select with firmware 4.17.9249
•  You can read and write CVs 15 & 16 but they have no effect

Locking Specifics

• Broadway Limited Paragon 2
• CV16=0 results in no lock regardless of CV15 value
• Will only take values 0-7. If you write a value >7, you get 0 when reading back the CV
• You can read any CV when decoder is locked but cannot write any CV except CV15
• ESU LokSound 5 DCC with firmware 5.6.143
• CV16=0 does NOT unlock the decoder regardless of CV15
• Only CV15=CV16 unlocks decoder
• Values from 0-255 work
• You cannot read or write CVs when locked except CV15
• NCE D14SR and DA-SR both with firmware version 3.5
• CV16=0 does NOT unlock the decoder regardless of CV15
• Only CV15=CV16 unlocks
• Values from 0-255 work
• You cannot read or write CVs when locked except CV15.

The author hopes this information will help model railroaders implement decoder locking when they feel it may be necessary. 

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