I'm referring to figure 66 on the datasheet of PGA117 (literature named SBOS424B–MARCH 2008–REVISED SEPTEMBER 2008). The figure illustrates the daisy chain read configuration with two PGA117 devices connected in the daisy chain configuration.
According to what I understood, the read command for the second device (called U2 in the fig) is clocked in to the DIN line of the first device (U1) during the first 16 clocks. During the next 16 clocks (clocks 17-32), this read command is clocked out on the DOUT of U1, which is clocked in via DIN of U2. During clock 17-32, the read command for U1 is clocked in through DIN.
Everything fine so far. My confusion arises from the next clock cycles (33-48). The figure shows the data from U1 appearing on its DOUT during this period. But, shouldn't the daisy chain read command given during clocks 17-32 to the DIN of U1 be clocked out through the DOUT during this period? Why is the data getting clocked out instead. Similarly, U2 received a daisy chain read commad during clocks 17-32. The figure shows the data from U2 being clocked out during clocks 33-48. Supposing I had a third device in the daisy chain, shouldn't U2 be clocking out the daisy chain read command it received previously through its DOUT.
Please explain how the daisy chaining behaves when there are more than 2 devices. Also, what exactly is the purpose of the NOP daisy chain instructions.