The quality of a block code is determined by its capability to protect data against undetectable errors and by the number of check bits that are required for that purpose. For a given number of check bits there are codes with optimum bit error detecting capability in shortened block lengths. These codes are determined and tabulated. The residual error characteristics of some of the tabulated codes are compared with those specified in ISO/CCITT or IEC standard data transmission protocols. For block lengths and bit error rates that are typical in process control applications, the residual error rate of the determined codes is more than six orders of magnitude smaller than that of codes specified by widely used standard transmission protocols.

Upgraded requirements for data integrity and data efficiency in real-time process control applications necessitate critical investigations on existing standard data transmission conventions such as the HDLC (high level data link control) protocol. It is shown that a single bit error within an ordinary HDLC frame may cause an undetectable message error at the receiver. Further proposals for using modified HDLC protocols which guarantee the detection of single bit errors fail to detect double bit errors within a frame. Refined assessments for corresponding nondetectable message error probabilities are based on bit sequence probabilities in HDLC frames which differ slightly from bit sequences in original text fields due to the `bit escaping mechanism' required by the HDLC protocol.