Packet Time Out Value
Summary: The Packet Time Out is an optional CV specified by the DCC Standard for Multifunction Decoders. The Packet Time Out value is stored in CV11. The standard suggests a minimum value of 20 seconds as the default.
Packet Time Out Value
For most multifunction decoders, the settings in CV11 determine how long the decoder will wait for a valid packet addressed to it, and if exceeded, the locomotive will stop.
As it is not a mandatory CV, manufacturers can choose to include it, or what it is used for.
Digitrax uses the value in CV11 to mute the audio in a sound decoder after a period of time elapses without any packets addressed to that decoder. The Soundtraxx Tsunami uses CV113 for controlling the mute function.
Digital Command Control Packet Structure
An NMRA Digital Command Control baseline packet consists of a minimum of two bytes:
- The Address Byte
- The Instruction Byte
The command station will calculate and append an Error Detection Byte. The packet will be transmitted with a preamble, the above two components, and the error byte.
The multifunction decoder reads all packets on the track. It will take the address and instruction bytes and calculate an error value. This will be compared to the error byte received from the command station. If they match, the decoder will check the address, and if it matches the value assigned to the decoder, it will then act on the instruction byte. If the comparison between the decoder's error byte and the packet's error byte do not match, the decoder will disregard the entire packet and wait for the next one.
If data corruption occurs due to various issues such as dirty track, poor wiring, or power bus issues, the multifunction decoder will determine the received packets are corrupt. It will continue to do whatever the last instructions told it to do.
If the DCC packets are damaged, the multifunction decoder will reject them. The packet time out determines how long a decoder will continue to operate in its current state. Should a loss of reliable data take place the decoder will time out and stop the train. The throttle has no ability to change that state until the data received by the multifunction decoder is valid.
Excessive time out values can become an issue. With a long time out, the locomotive could derail, crash, or end up taking a dive to the floor.
When using Energy Storage devices, a short timeout is advised, to prevent loss of control should the DCC signal be lost for an extended period of time.
Command Station Issues
Digtrax Sound Decoders
Some command stations will not create packets for any address whose speed is zero (stopped). In cases like this, a Digitrax sound decoder would mute when the locomotive came to a stop. Setting CV11 to 0 prevents that.