The diodes functions are as follows. The top two diodes in the diagram are used to make a logic function of SSTB be high level when both SCLK and SDAT are high. If either of these is low the SSTB will be low.

The third and lowest diode makes the data out line from the 4021 input shift register be effectively "open drain". You cannot tie a pushpull output from a chip right to the SDAT line.

All that said however, note that the use of diodes like this in a circuit is not the best concept. They add about 0.7 volts to the V_OL seen on the down wind side of the diodes. It will work OK if all interfaces involved are CMOS threshold levels at 1/2 V_CC. When you get into the realm of circuits that have a V_IL requirement that is compatible with TTL at 0.8 volts then the use of the diodes as shown virtually erases any low level noise immunity the circuit may have had without them.

If you have to build a circuit with diodes I would reserve such use for hobby projects or one shot lab expiriments. And even then I would recomment using Shottkey diodes like a BAT54 that have a drop of 0.3 -> 0.4 volts.

Michael Karas