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I suspect this may be a problem of the timing of publishing and setting up the listeners.

A listener object takes a short period of time to attach to a topic before it will start receiving messages. in Node1 you are publishing on the topic chatter1 then then subscribing to the topic for the expected reply c. It is possible that the subscriber is not set up in time for it to catch the reply sent by Node2.

I recommend moving the line rospy.Subscriber('c', String, callback2) before the publish commands and adding a 1 second delay after it to make sure it's setup before you publish the test messages.

Let us know if this fixes your problem.