Described are communication systems that convey differential and
common-mode signals over the same differential channel. Noise-tolerant
communication schemes use low-amplitude common-mode signals that are
easily rejected by differential receivers, thus allowing for very high
differential data rates. Some embodiments employ the common-mode signals
to transmit backchannel signals for adjusting the characteristics of the
differential transmitter. Backchannel control signals are effectively
conveyed even if the forward channel transmitter is so maladjusted that
the received differential data is unrecognizable. Systems in accordance
with the above-described embodiments obtain these advantages without
additional pins or communications channels, and are compatible with both
AC-coupled and DC-coupled communications channels. Data coding schemes
and corresponding data recovery circuits eliminate the need for complex,
high-speed CDR circuits.