A bidirectional repeater and data multiplexer for serial data has A-side I2C port devices A1-A4 coupled to comparators 302-308 and pulldowns to ground 316-322. Comparator outputs are coupled responsive to select lines S1-S4 of N:l Select 310 to terminal A1 of bidirectional control 210 to control pulldown to non-zero low voltage Vp 206 at B-side device B. An inverting comparator 208 coupled to terminal B1 of bidirectional control 210 responds to input threshold voltage Vt less than low voltage Vp, to prevent data lockup due to data flowback to devices A1-A4. Output data from comparator 208 is coupled responsive to select lines S1-S4 of 1:N Select 312 to control pulldowns 316-322. This selectively repeats routing of device A1-A4 data to device B. Data from device B is selectively routed to pulldowns of devices A1-A4.