The 4951B is a complete field service data communications Protocol Analyzer. It is small, lightweight and easy-to-use. It contains the essential features needed to verify proper data equipment installation and maintenance. It correctly diagnoses system operating problems and assist in their correction. The Protocol Analyzer can simulate network components, monitor data transmission, and perform bit error tests. Data analysis may be done real time or in the post-processing mode. The 32 Kbytes of non-volatile memory stores all data, timing, and lead status information. Additional nonvolatile memory keeps all test setups as well as monitor and simulate programs. For long term storage of data and important test programs, the optional mass storage cassette tape (option 001) stores an additional 512 Kbytes of information. Six software keys provide operator interface to the extensive troubleshooting capability of the 4951B. Whenever an instuction indicates use of a softkey, its mnemonic will be enclosed by <>. The user friendly interface, combined with auto-configuration of all data link parameters, allows many complete tasks to be performed quickly and easily, resulting in less network downtime.