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.
"While in digital operations mode each Multifunction Digital Decoder shall have a Packet Update timeout value. While in digital operations mode, if the packet time-out value is exceeded, the multifunction Digital Decoder will bring to a stop all controlled devices. The purpose of this time-out is to insure that each Multifunction Digital Decoder receives a periodic update from the Digital Command Station and thereby help prevent runaway conditions."
- A Value of Zero (0) disables the timeout.
Command Station Issues
Some multifunction decoders are configured at the factory with a non-zero value in CV11.
- QSI multifunction decoders often had a non-zero value in CV11
- 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 disables the timeout.
- A known issue is the CVP EasyDCC system: It only transmits packets when a change occurs to a specific multifunction decoder address. If no functions or throttle changes occur the locomotive will come to a stop. Pressing a function button or changing the throttle causes the locomotive to begin moving. While it reduces traffic on the DCC bus and makes things happen instantly, this feature can confuse operators. Adjusting the Packet Timeout Value will correct this issue.
In addition to S-9.2.4, the following NMRA Standards and Technical notes provide more dietails:
- TN-188.8.131.52: Advanced Extended Packet Formats; May 15, 2022
- S-9.2.2: Configuration Variables For Digital Command Control, All Scales, July 2012
- S-9.2.4: Fail-Safe Operating Characteristics For Digital Command Control, All Scales, July 2012