The DOUT module uses multiple serial registers 74HC595 (not included w/ pcb); every register provides 8 digital outputs, which are updated with the latch enable signal RCLK and shifted out with the clock signal SCLK. The advantage of using such shift registers is, that they can be cascaded to a long chain without the need of more than 5 cables to the core module. The DOUT board has been designed on a way which allows to cascade not only the ICs on the board, but also several DOUT modules. But note that it depends on the firmware how much outputs are processed by the core module:

• MIOS8 based applications typically allow to cascade up to 4 modules (=16 DOUT SRs), makes up to 128 digital outputs
• MIOS32 based applications typically allow to cascade up to 8 modules (=32 DOUT SRs), makes up to 256 digital outputs

Some applications also allow to scan the shift registers in a so called "matrix" configuration. E.g. a 8x8 matrix (which is driven by a two DOUT SRs) can drive up to 64 LEDs, a 16x16 matrix could drive up to 256 LEDs. But normally 8x8 matrices are perferred due to higher duty cycle: 8x8 matrices make LED visible at 12.5% of the time which is normally bright enough, 16x16 matrices make LED visible at only 6.2% of the time which is mostly too dark!.