Peritek VCD-V, User Manual

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Peritek
5-64 Programming On-board Devices and Memories
5.12 Serial I/O Ports (DUART)
The DUART contains four independent asynchronous serial I/O ports.
Each port can be programmed separately for transmit and receive baud
rates, with a maximum baud rate of 38 Kb. The receive buffers are
quadruply buffered, to minimize the possibility of data overrun. The serial
interface provides data-leads only RS-232 as well as internal loopback (for
testing). To obtain more understanding of the DUART and its many
programmable functions, please refer to the 2681 data sheet available from
the manufacturer (see Section 1.2 or contact Peritek)
The DUART uses a 3.6864 MHz oscillator for its master clock. Each
DUART has internal divider chains provide a full range of software
programmable baud rates and timer periods.
44-pin DUARTs are used on the graphics boards. Some control lines are
reserved for HSP operations (see Section 5.15) and for programming the
serial EEPROM (see Section 5.17). Unused inputs are connected to
pullups and jumper pin blocks for use as user inputs. Three control outputs
are used to drive red, yellow, and green LEDs.'
The RS-232 interface is provided by a MAX238 CMOS quad EIA RS-232
receiver/transmitter. This chip provides four complete channels as well as
built in slew rate control. The chip also in/- 10 volt charge pump
generators to supply the necessary RS-232 voltage swings and clamping
diodes for protection against static charges on both inputs and outputs.
The DUART contains 16 register locations, several of which are either
read or write only. The register number shown in the table below is added
to the DUART base address (in the device buffer) to obtain the actual
location. The values enable TX, RX, no parity, 8 bits/char, no loop, no
RTS or CTS, 1 stop bit, 38.4 KB based on external clock (3.6864 MHz),
and no interrupts.
The DUART has a single interrupt request line which is connected to the
34020 X2P interrupt pin. Internally, an interrupt can be caused by break,
receiver full, and transmit ready for either channel . Externally, an
interrupt can be caused by one of the two DUART input lines which
function as change of state interrupts from the PC Keyboard controller.
The interrupt mask register (IMR) filters out unwanted interrupts. A
complementary interrupt status register holds the ANDs of the request(s)
and the corresponding IMR bit. The following table lists the registers and
some sample values.