The TRDP protocol defines the exchange of MD (Message Data) between a Caller and one or more Repliers on a confirmed TRDP service. Since it is only used for real-time message data, the length of the message is limited to 64 kb.
Roles
In the process of Message Data interaction, there are two roles:
•Caller: The source device of MD messages in the Push communication mode; the destination device of MD messages in the Pull communication mode.
•Replier: The destination device of MD messages in the Push communication mode; the source device of MD messages in the Pull communication mode.
TRDP provides three MD transmission methods:
•The request message has no response ('notification')
•The request message has a response but no confirmation ('request without confirmation')
•The request message has a response and confirmation ('request with confirmation')
Push communication mode
It supports various modes such as point-to-point, point-to-multipoint, the source device knows the destination device, and the source device doesn't know the destination device.
Pull communication mode
It supports various modes such as point-to-point, point-to-multipoint, the destination device knows the source device, and the destination device doesn't know the source device.
MD message format
The IEC standard defines the MD message format as follows:
|
Field |
Length |
Value |
Description |
|
SequenceCounter |
4 bytes |
Counting starts from 0 |
|
|
ProtocolVersion |
2 bytes |
Fixed value |
High byte：MainVersion Low byte：SubVersion For example: '0102' = Version 1.2 |
|
MsgType |
2 bytes |
‘4D6E‘H (‘Mn’) ‘4D72‘H (‘Mr’) ‘4D70‘H (‘Mp’) ‘4D71‘H (‘Mq’) ‘4D63‘H (‘Mc’) ‘4D65‘H (‘Me’) |
‘Mn’ = Notification (Request without reply) ‘Mr’ = MD Request with reply ‘Mp’ = MD Reply without confirmation ‘Mq’ = MD Reply with confirmation ‘Mc’ = MD Confirm ‘Me’= MD error |
|
comId |
4 bytes |
Set by the user |
Identifier of user data, supporting Unspecified message (ComId = 0) |
|
etbTopoCnt |
4 bytes |
0..-1 |
The ETB topography counter: •Used as the train's address in IEC 61375-2-5 (parameter ‘etbTopoCnt’) •Set by the user •Must be set for all ETB-based communications •If a valid opTrnTopoCnt is set, this field must also be set •Set to 0 in other cases |
|
opTrnTopoCnt |
4 bytes |
|
The operational train topography counter: •Refer to the description in Section 5.3.3 for usage •Set by the user •Set this field when information in the operational train directory is required (e.g., routing operations) •Set this field when the source device uses the operational train dictionary to resolve the destination IP (e.g., resolving the URI ‘vcu.leadVeh.anyCst.anyClTrn.lTrn’) •Set to 0 in other cases |
|
DatasetLength |
4 bytes |
0 ..65388 |
Length of user data, in bytes, excluding padding bytes |
|
ReplyStatus |
4 bytes |
< 0: NOK 0: OK > 0: user reply status |
Set when the Replier reports the execution result of the request message, or when the caller sends a confirmation message. The Replier carries this field in the reply message. –1 – reserved –2 – session abort –3 – no replier instance (at replier side) –4 – no memory (at replier side) –5 – no memory (local) –6 – no reply –7 – not all replies –8 – no confirm –9 – reserved –10 – sending failed |
|
SessionId |
16 bytes |
Calculated |
•Can identify a set of "request-reply" session or "request-reply-confirm" session •Can identify a set of "request-error" session |
|
ReplyTimeOut |
4 bytes |
1..-1 0==Infinite time |
Used in request/reply Set to 0 in 'Mn', 'Mp', 'Mc' and 'Me' |
|
SourceURI |
32 bytes |
|
•For addressing function •It is a null-terminated string •Padded with 0 at the end •The URI only contains the user part, excluding the host part and @ •It may be an all-0 empty string |
|
DestinationURI |
32 bytes |
|
•For addressing function •It should be a null-terminated string •Padded with 0 at the end •The URI only contains the user part, excluding the host part and @ •It may be an all-0 empty string |
|
HeaderFCS |
4 bytes |
|
•Calculated based on the PD-PDU header •Exclude the headerFcs itself during calculation |
|
Dataset |
0..65388 |
|
Must be an integer multiple of 4 bytes. Pad with zeros if insufficient. |
