IBM RT Series Hardware Reference Manual page 82

Bit 4 This bit provides a loopback feature for diagnostic testing of the NS16450. When bit 4
Bits 5-7
is set to logical 1, the following occurs:
The transmitter serial output (SOUT) is set to the marking (logical 1) state.
The receiver serial input (SIN) is disconnected.
The output of the transmitter shift register is "looped back" into the receiver shift
register input.
The four modem control inputs (-CTS, -DSR, -RLSD, and -RI) are disconnected.
The four modem control outputs (-DTR, -RTS, -OUT 1, and -OUT 2) are
internally connected to the four modem control inputs, and the modem control
output pins are forced high.
In the the diagnostic mode the receiver and transmitter interrupts are fully operational.
The modem control interrupts are also operational, but the sources of the interrupts are
now the lower 4 bits of the modem control register instead of the 4 modem control
inputs. The interrupts are still controlled by the interrupt enable register.
The NS 16450 interrupt system can be tested by writing into the lower 6 bits of the line
status register and into the lower 4 bits of the modem status register. Setting any of
these bits to a logical 1 generates the appropriate interrupt (if enabled). The resetting
of these interrupts is the same as in normal NS 16450 operation. To return to normal
operation, the registers must be reprogrammed for normal operation and then bit 4 of
the modem control register must be reset to logical O. The transmitter should be idle
when this bit changes state.
These bits are permanently set to logical O.
26 5080 Peripheral Adapter