We modify the maximum a posteriori (MAP) detection and develop the soft-input soft-output (SISO) MAP detection to apply to iterative differentially coded systems. Not only the forward but also the backward SISO MAP detection is introduced to provide more information based on the channel symmetry. Since the MAP detection has high complexity, the truncated SISO MAP detection is used to reduce the complexity. To enlarge the entire detection length and enhance the performance, the SISO FB_MAP detection consists of truncated forward and backward SISO MAP detection. The proposed scheme performs better than the a posteriori probability (APP) demondulator scheme with the same memory requirement.