Abstract: Apparatus for controlling RTS/DCD simulation in a data receiver by detecting, after the received data transmitted for assignment to a particular data terminal, the presence of a two-part code having a repetitive stream of binary 1's or 0's (or repeating sequences thereof) as the first part; and for thereafter detecting the presence of a preselected coded sequence of bits within a preselected number of succeeding bits received as the second part of said two-part code. The preselected two-part code is advantageously chosen to be one whose probability of occurrence in customer data is extremely remote.