Some long winded explanation by way of analogy:
- Programming on the main (POM) / Ops mode programming: Dear Mr. Loco number 3, Please change this CV for me please. I hope you can hear me but I know you cannot respond and confirm that you did what I asked you to do. This happens under full track power. As long as you can control the loco in some fashion using a loco address: blink the headlights, move the loco around. The chances are very high this method will be successful even though you have no way of getting any sort of direct confirmation that the CV was actually changed. If you go back in to the SAME CV again using POM, you MIGHT see the value that you put in previously. Note**** You are NOT reading it from the Decoder! The system is only remembering that last command you did.
- Program Track mode also known as Service Mode Programming: When you enter into this mode a sound decoder SHOULD shut the sound off!! If it does not do that, then the chances that it is also actually listening and responding to your commands are very slim. Service Mode does not know or care which loco is on the tracks. This method is: send a command / wait for a response. This method uses a maximum of 250 mA track power and will detect short circuits. Not all decoders support this mode. Do not assume that your decoder supports this mode unless the manufacturer expressly says so. If you attempt to Read a CV, you should get a value or you will get an error message like “Cannot Read CV”. If you attempt to change a CV, the decoder should give you some sort of response in return. This response can be a flashing light, the loco will click, or the loco will move briefly. This is the locos method of waving a hand and saying “ heard your request and tried to do what you asked”. You should then be able to go in and verify that the changes were made.