Diagnostics <D ACK> Command

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If you encounter problems with ACKs from the Decoder (Reading or Writing CVs) and you want help, the DCC-EX support team will ask you to provide a log. This is a very simple diagnostic test to provide us with the proper information. With your loco on the PROG track, and using a serial monitor like the one in the Arduino IDE, enter each of these two commands followed by pressing “send”:


This will turn ACK diagnostics ON and then try to read the appropriate CVs to determine your loco address. If you don’t see your loco address at the end of the report (it could be incorrect or <r -1>), send us the log (see below for an example), and we can help you diagnose the problem.

More Detail

When the ACK processing on the prog track does not work as expected, you may want to use the <D ACK ON> command in a serial command window. This is an example how to read CV8 with diagnostics on. You enter the first 2 lines, <D ACK ON> followed by send, then <R 8 1 1> followed by send. DCC-EX does not echo what you write, but your serial command window may. With diag on you get the extra 11 lines of output compared to if you just entered the command with diagnostics off. The last line is the answer, CV8=145:

<R 8 1 1>
V0 cv=8 bit=7 NO-ACK after 143mS max=12/35mA pulse=0uS
V1 cv=8 bit=7 ACK-OK after 34mS max=646/1931mA pulse=6852uS
V0 cv=8 bit=6 ACK-OK after 34mS max=583/1743mA pulse=6844uS
V0 cv=8 bit=5 ACK-OK after 34mS max=646/1931mA pulse=6800uS
V0 cv=8 bit=4 NO-ACK after 144mS max=12/35mA pulse=0uS
V0 cv=8 bit=3 ACK-OK after 34mS max=563/1683mA pulse=6792uS
V0 cv=8 bit=2 ACK-OK after 34mS max=647/1934mA pulse=6800uS
V0 cv=8 bit=1 ACK-OK after 34mS max=573/1713mA pulse=6844uS
V0 cv=8 bit=0 NO-ACK after 145mS max=14/41mA pulse=0uS
VB cv=8 value=145 ACK-OK after 34mS max=639/1910mA pulse=6848uS
<r1|1|8 145>

Your output may show different formatting with respect to linefeeds. Here the decoder reports that bits 7, 4 and 0 are NOT Zero. That gives the value of 10010001 = 145 which is the manufacturer ID for Zimo. That value is checked in the VB line. For a successful read, the result of the verify bit and verify byte commands have to match, otherwise the value can not be read. If you have problems reading decoders, you can compare the received values with the expected values. The format is:


The CURRENT should be over 60mA for a successful ACK and the length should be 6000µS +-1000µS but because of Decoder variations from the standard, DCC-EX has some extra pulse length margin.

In this example, we are checking CV 8, which is the manufacturer ID for your decoder. We then check each of the 8 “bits” in the “byte” that holds the value in that CV.

The first test, ACK-BASELINE gets a baseline reading of the current on your programming track with the loco just sitting there.

Since most bits will be 0, we check that first to save time. V0 means Verify zero. If do not get a zero, we then try to verify a 1 - V1 means “Verify one”. If we don’t get either, we display an error. A bit must be either a 0 or a 1 so the test will fail, but the data returned can help us see why.

If we succeed on the first bit, we check each of the remaining 7 bits. NO-ACK means we did not see the bit value we were testing for, ACK-OK tells us we received an ACK. After each bit test, we show how long it took to receive the ACK (or the timeout value if we detected none), the Arduino raw pin reading being sent by your motor controller’s current sense circuit, and what that translates to in milliAmps. Finally, we report the duration of the ACK Pulse, if detected.

We do one final test at the end to Verify Byte, you see that as VB. This does a double-check to see if the byte contains what we found by checking it one bit at a time. The must match for a successful read.

Other than when using the “decoder address test”, <R> with no parameters, you need to enter CV read commands with all 3 parameters. The format is <R CV x y> where R stands for read and CV is the CV number you want to check. The X and Y values can be anything, but must be entered. They are an advanced feature for programmers whose software can work with DCC-EX (like JMRI). So you would enter <R 8 55 55> or R 8 1 1> to try and read CV 8. The response is <r CV x y> where “x” and “y” are whatever numbers you entered after the CV value.

To turn off the ack diagnostics use any parameter that is not “ON” or “LIMIT”.

<D ACK NOPE>, <D ACK OFF>, etc.

Diag messages off.


The Ack current limit is set according to the DCC standard(s) of 60mA. Most decoders send a quick back and forth current pulse to the motor to generate this ACK. However, some modern motors (N and Z scales) may not be able to draw that amount of current. You can adjust down this limit. Or, if for some reasons your acks seem to be too “trigger happy” you can make it less sensitive by raising this limit.


would set the ack limit to 30mA (more sensitive).


would set the limit to 100mA (less sensitive).

The custom ack limit will be effective until you restart the Command Station (it will not “stick” in EEPROM). If you wish to permanently set the ACK LIMIT, you may enter it as a command in the mySetup.h file.



The NMRA specifies that the ACK pulse duration should be 6 milliseconds, which is 6000 microseconds (µS), give or take 1000 µS. That means the minimum pulse duration is 5000 µS and the maximum is 7000 µS. There are many poorly designed decoders in existence so DCC-EX extends this range from 4000 to 8500 µS. If you have any decoders that still do not function within this range, you can adjust the ACK MIN and ACK MAX parameters.

Example 1: You use the <D ACK ON> and <R> commands described above to generate a test log from a loco on your programming track. You see that there are ACK pulses but that their durations are less than 4000 microseconds. The log shows the shortest one was 3450. You might then choose a setting a little bit lower than your lowest reading:

<D ACK MIN 3300>

Example 2: You use the <D ACK ON> and <R> commands described above to generate a test log from a loco on your programming track. You see that some of the ACK pulses are longer than 8500 µS. The longest one is 10350 µS. Pick a setting a little bit higher than your highest reading like this:

<D ACK MAX 10500>


<D PROGBOOST> - Override 250mA prog track limit while idle.

When the programming track is switched on with <1> or <1 PROG> it will normally be restricted to 250mA according to NMRA standards. Some loco decoders require more than this, especially sound versions. <D PROGBOOST> temporarily removes this limit to allow the decoder to use more power. The normal limit will be re-imposed when the programming track is switched off with <0> or <0 PROG> or the Command Station is reset.

Making the Changes Permanent

To make these settings permanent, you can create a Startup Configuration by adding or editing an existing mySetup.h. You simply add the appropriate commands in the correct format and upload the Command Station software again. Click on the link in this paragraph for detailed instructions.