The control unit is arranged on the knotter arm between the swivel bearing of the arm and the knotter head. A single toothed segment is arranged for the drive to the pinion. By the pivoting motion of the knotter arm the pinion comes into the circular path of the segment consequent upon the control drum rotation. The knotter moves in an arc the radius of which is the total length of the knotter arm by which the movement is nearer to a straight line than with known makes. The carrying of the control unit in the centre area of the knotter arm leads to an increased compounding of the control unit movement on that of the knotter. This, apart from advantages in cost by the reduction in size of the cam and the control drum, has the advantage in reliability.