Technical information

Serial Solutions NewBIOS Reference
bit 6 = RI, Ring Indicator
bit 5 = DSR, Data Set Ready
bit 4 = CTS, Clear To Send
bit 3 = Delta DCD, set true if bit 7 has
changed since last read
bit 2 = Delta RI " " " " 6 " " " " "
bit 1 = Delta DSR " " " " 5 " " " " "
bit 0 = Delta CTS " " " " 4 " " " " "
These return values for AL, AH are the same as for
services 0H, 3H and 4H (they are generated by the same piece
of code).
BL is Modem control register, as above.
Meaning._________
Service 5H allows the user to read and set the output lines
from the 16450 and the loop function. Subservice 0, where AL
is 0, reads the modem control register back into BL. Subservice
1, where AL is 1, writes the value in BL into the modem
control register.
Other Points.____________
From the point of view of the 16450 the four output lines
are identical, but serial cards usually use them as follows.
Bit Use
DTR Becomes the DTR signal on the
RS232 interface
RTS Becomes the RTS signal on the
RS232 interface
Out1 Not used
Out2 When set TRUE gates interrupts
from the 16450 ON.
The last line, out2, is worth examining in greater detail. When
the out2 bit is set true logic on IBM’s serial card allows the
Page 88 Chapter 5