This paper describes an analysis for the probability of undetected errors for SDLC/Manchester encoded links. Results of mean time between undetected errors versus probability of line error rate are given. Such graphs are of interest to free space IR link designers offering better insight into the robustness of their design, for 16 or 32 bit CRCs. Coupled with the cost constraints it allows one to decide, with increased confidence, on the suitability of the combination of modulation scheme, framing and choice of CRC for the product under consideration. Furthermore the analysis offers a means for determining the weakest part (most susceptible to noise) of the SDLC framing.