Figure 6.

Schematic design of a generic modular multi-bit counter

The counter employs three types of modules, each implemented in a different cell type. One connector of type 1 and N$-$ 2 connectors of type 2 (colored in dark and light green respectively) are shown in the first row and N single-bit counters (colored for different bit levels gradient blue) are in the second row. The external input signal is colored in dashed purple lines, and it acts on all connectors simultaneously. Black arrows denote communication via diffusible small molecules. Each connector outputs two signals $C_{i,1}$, $C_{i,2}$ at ith bit level. The $C_{i,1}$ is the AND operation result between the previous bit's output and the previous connector's output. The $C_{i,2}$ is the AND operation result between the $C_{i,1}$ and the external signal.