OTDR - Z80 instruction reference

otdrOutput, decrement and repeat 
Repeats OUTD, writing bytes from memory downward from HL to the port at BC until B is zero.
2 bytes · 21/16 cycles·21 cycles for a pass that repeats, 16 for the last
SZHP/VNC ?1??1?
1 always 1
? undefined afterwards
Forms  otdr  
B is decremented. A byte from the memory location pointed to by HL is written to the port at the 16-bit address contained in the BC register pair. Then HL is decremented. If B is not zero, this operation is repeated. Interrupts can trigger while this instruction is processing. Note that the carry flag may be affected, contrary to documentation. Read More
         .org 0x8000
        ld hl, source + 1
        ld b, 2
        ld c, 1
        otdr
        halt
source: .byte 42, 7