Overview
The rosserial protocol is aimed at point-to-point ROS communications over a serial transmission line. We use the same serialization/de-serialization as standard ROS messages, simply adding a packet header and tail which allows multiple topics to share a common serial link. This page describes the low-level details of the packet header and tail, and several special topics used for synchronization.
Packet Format
1st Byte - Sync Flag (Value: 0xff) 2nd Byte - Sync Flag (Value: 0xff) 3rd Byte - Topic ID - Low Byte 4th Byte - Topic ID - High Byte 5th Byte - Message Length (N) - Low Byte 6th Byte - Message Length (N) - High Byte N Bytes - Serialized Message Data Byte N+7 - Checksum
Topics ID 0-100 are reserved for system functions, as defined in the rosserial_msgs/TopicInfo message.
The checksum is used to make sure that a particular packet has not been corrupted. It is computed as follows:
255 - ( (Topic ID Low Byte +
Topic ID High Byte +
Message Length Low Byte +
Message Length High Byte +
data byte values) % 256)
Topic Negotiation
Before data transfer can begin, the PC/Tablet side must query the Arduino or other embedded device for the names and types of topics which will be published or subscribed to.
Topic negotiation consists of a query for topics, a response with the number of topics, and packets to define each topic. The request for topics uses a topic ID of 0.
The query for topics will look like:
0xff 0xff 0x00 0x00 0x00 0x00 0xff
A series of response packets (message type rosserial_msgs/TopicInfo, each containing information about a particular topic, with the following data in place of the serialized message:
uint16 topic_id string topic_name string message_type string md5sum int32 buffer_size
Here, the topic name is the name of the topic, for instance "cmd_vel", and message type is the type of the message, for instance "geometry_msgs/Twist".
If a response packet is not received correctly, another query may be sent.
New in 0.3.0 MD5 checksums are now transmitted to verify that both the sender and receiver are using the same message.
Time
Time synchronization is handled by sending a std_msgs::Time in each direction. The embedded device can request the current time from the PC/Tablet by sending an empty Time message. The returned time is used to find clock offset.