74HC123 in figure 15 (as below) is play a role of state machine. It watches for transitions on /RO. Once detected the start bit (high to low, here should be low to high), means there is a data-byte transfer is coming, 74HC123 will enable the Tx of MAX1480B (driver enable), and provide output at A/B. The R(51k) and C (1000pF) deiced the Tx enable period. So it should be larger than one data-byte transmission time. at the same time, it should be as small as possible and try to leave the bus available for other Tx.