A basic problem with the program is that a single motor is being controlled by two different threads. Threads should do different things. For example you might have threads that play music, operate an arm, and operate drive motors simultaneously. Two NXT threads should not control the same sensor, motor, or output, however. Cooperative concurrency (where threads share resources) is the hardest part of parallel programming, and Mindstorms NXT really isn't suitable for this degree of complexity. Here's a good tutorial on using threads for anyone who's interested: http://www.nxt-tutorial.stemcentric.com/Threading.html Ralph On 10/23/12 9:11 PM, Bob and Ann Henshaw wrote: > > Here is a simple program where the robot moves, stops on a line, > executes a 2 motor 90 degree spin, and moves forward. At the end, it > suddenly lurches to the right, despite the stop block. > > Thanks for looking at it! > > Bob > > ... -- Ralph F Grove, Ph.D. Professor James Madison University Department of Computer Science -- To UNSUBSCRIBE or CHANGE your settings, please visit https://listserv.jmu.edu/archives/vadcfll-l.html and select "Join or leave the list". -- VADCFLL administrative announcements are sent via VADCFLL-ANNOUNCEMENTS-L. Visit https://listserv.jmu.edu/archives/vadcfll-ANNOUNCEMENTS-l.html to subscribe.