The Oscom Nano uses one CD4051B single 8 channel multiplexer and one CD4042B quad clocked D latch chip to decode a max of 16 keys. The CD4042B chip is used as buffers to 'store' the 4 lower bits of OUT 2. The lower 3 bits (0 to 2) are used as input to the CD4051B and bit 3 is used as the NY1 and NY0 (inversed bit 3) of the matrix. The 8 outputs of the CD4051B with the 2 NY lines create the matrix for 16 keys. The CD4051B is also connected to EF3 to give the 1802 the possibility to check if the key representing the last OUT 2 is pressed.
The CD4556 is used to activate the CD4042B on an OUT 2 instruction.
For the exact schematics of the keyboard hardware see figure 1.