Specifications

Serial Communications (82050)

Modem Control

The Modem Control register, shown in Figure 11-4, includes the

serial handshake, RS-485 driver enable, and parallel port enable

signals.

Address:03FCh

Access:Read/Write

76543210

DTR

Data Terminal Ready

0 DTR = mark

1 DTR = space

RTS

Request To Send

0 RTS = space

1 RTS = mark

OUT2

Output 2

0 Parallel ports disabled

1 Parallel ports enabled

LOOP

Loopback Diagnostics

0 Disabled

1 Enabled

000

Figure 11–4. Modem Control Register.

The DTR bit (bit 0) defines the state of the Data Terminal Ready

(DTR) signal. Programming the DTR bit with a logical 0 forces DTR

to a marking (negative) state, and programming the DTR bit with a

logical 1 forces DTR to a spacing (positive) state.

The DTR signal can be jumpered to enable and disable the RS-485

transmit data and request to send drivers if the ACC serial port is

configured for RS-485 operation. In this configuration, programming

the DTR bit with a logical 0 disables the drivers, and programming

the DTR bit with a logical 1 enables the drivers. The power-on state

of DTR disables the drivers.

11-13