DTC P1600: Serial Communication Problem Between ECM & TCM: Description
WARNING: This page is about a different car, the 2002 Suzuki Esteem. However, it is still accessible from the selected car via links, so may be relevant.
The serial data line is pulled up to about 5 volts by Engine Control Module (ECM). Transmission Control Module (TCM) transmits information to ECM through serial data line by controlling circuit ground. When ignition is on, TCM constantly sends information to ECM.