A technique used with alternate mark inversion (AMI) to support the transmission of long strings of zero bits, which are not unusual in data communications applications. When B8ZS encounters a string of eight zeros (00000000), it substitutes a specific bit pattern that intentionally violates AMI, i.e., includes an intentional bipolar violation (BPV). If the preceding mark (1 bit) was represented as a +3V (positive 3 volts), the substituted bit pattern is 0 0 0 +