DSP 104-plus, Справочное руководство

Страница: 32 / 140

22 158004.B00
Connection is made to the COM3 and COM4 serial ports via the 14-way J5
connector.
The pin assignments of the first three serial ports are such that they easily connect to
9-pin D-type connectors.
The first three serial ports provide the full complement of RS-232 signals. Transmit
Data, Request To Send (RTS) and Data Terminal Ready (DTR) are outputs from the
TP400. Receive Data, Data Carrier Detect (DCD), Data Set Ready (DSR), Clear to
Send (CTS) and Ring Indicator (RI) are inputs to the TP400. COM4 provides only
Transmit Data and Receive Data, and as TTL-level signals. The control inputs of
COM4 are all connected so as to appear permanently asserted.
Following a reset of the TP400 the serial ports are initialized as 2400 baud, one stop
bit, eight data bits and no parity. These parameters can be changed by the MS-DOS
MODE command.
COM1 serial port uses interrupt level IRQ4 to interrupt the processor. The COM2
serial port uses interrupt level IRQ3. COM3 and COM4 use interrupt levels IRQ5 and
IRQ9 respectively. Note that in some PC systems with four serial ports COM3 shares
an interrupt with COM1 and COM4 shares an interrupt with COM2. The TP400
design allows each serial port to have its own interrupt. (See section 4.5 for
information on re-allocating interrupts).
It should be noted that the BIOS does not make use of serial port interrupts, but that
most comms software packages enable the interrupts and make use of them to
increase the speed of serial data transfer.
3.4.2 RS-485 Operation
As an option COM2 can be re-configured as an RS-485 serial port. This is done with
a solder link on the board - see Appendix B for configuration details.
The COM2 RS-485 port configuration provides either half-duplex or full duplex
interfaces. In full duplex mode one twisted pair is used for transmission and another
twisted pair is used for reception. Full duplex mode would normally be used in point-
to-point communication between two computers.
In half duplex mode the transmit and receive twisted pairs are connected together at
the TP400. In this mode several boards can be connected to the single twisted pair,
with no more than one board driving the cable at once. A suitable protocol needs to
be agreed by all nodes on the twisted pair to ensure that only one computer transmits
at any one time.
On the TP400 the RS-485 driver is controlled by the RTS bit of the on-board UART.
When RTS is off (inactive) the RS-485 transceiver chip does not drive the transmit
twisted pair cable. This is the default state after a TP400 reset. When RTS is set
active the RS-485 transceiver does drive the transmit twisted pair cable and the
TP400 can transmit. Note that the receiver part of the transceiver is always enabled.
Thus in half duplex mode COM2 will receive the characters that it transmits itself.