The 3781B Pattern Generator allows binary or code errors to be injected as single shot or at 10E-03 or 10E-05 rates into a wide range of PRBS and 16-bit WORD test patterns coded in AMI or HDB3. The test patterns provided include standard 2E+09 - 1, 2E+15 - 1, and 2E+23 - 1 bit PRBS to CCITT recommendations, fully programmable 16-bit WORD, and two 8-bit WORDS which may be alternated under the control of an external signal.