IBM RT Series Hardware Reference Manual page 447

Bit 0 This bit controls the '-data terminal ready' ( -DTR)
output. When bit 0 is set to logical 1, the -DTR
output is forced active. When bit 0 is reset to logical
0, the '-DTR' output is forced inactive.
Bit 1 This bit controls the '-request-to-send' (-RTS)
output. Bit 1 affects the' -RTS' output in the same
way bit 0 affects the '-DTR' output.
Bit 2 This bit controls the '-Output 1 ' (-OUT 1) signal,
which is a spare the programmer can use. Bit 2
affects the '-OUT 1 ' output in the same way bit 0
affects the '-DTR' output.
Bit 3 This bit controls the '-Output 2' (-OUT 2) signal,
which is a spare the programmer can use. Bit 3
affects the '-OUT 2' output in the same way bit 0
affects the '-DTR' output.
Bit 4 This bit provides a loopback feature for diagnostic
testing of the controller. When bit 4 is set to logical
1, the following occur: the 'transmitter serial output'
(SOUT) is set to the active state; the 'receiver serial
input' (SIN) is disconnected; the output of the
transmitter shift register is "looped back" to the
receiver shift register input; the four modem-control
inputs ( '-CTS " '-DSR', '-RLSD', and '-RI') are
disconnected; and the four modem-control outputs
('-DTR', '-RTS', '-OUT l' and '-OUT 2 ') are
internally connected to the four modem control
inputs. In the diagnostic mode, data sent is
immediately received. This feature allows the
processor to verify the transmit- and receive-data
paths of the controller.
In the diagnostic mode, the receiver and transmitter
interrupts are fully operational, as are the
modem-control interrupts. But the interrupts'
sources are now the lower four bits of the modem
control register (MCR) instead of the four
modem-control inputs. The interrupts are still
controlled by the interrupt enable register.
Serial/Parallel Adapter 13