Automatic Rs-485 Tx Enable; Adding Serial Ports; Com3 And Com4 Interrupt Configuration Register - Technologic Systems TS-5700 User Manual

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TS-5700 User's Manual

5.4 Automatic RS-485 TX Enable

TS-5700 boards support fully automatic TX enable control. This simplifies the design of half-duplex systems

since turning off the transmitter via the RTS signal is typically difficult to implement. The COM1 UART transmit

holding register and the transmit shift register both must be polled until empty before deasserting RTS when

using the RTS mode. The design gets more difficult when using the TX FIFO or when using a multi-tasking OS

such as Linux.

In Automatic mode, Timer2 and a Xilinx PLD keep track of the bits shifting out the COM1 UART. This circuit

automatically turns on/off the RS-485 transceiver at the correct times. This only requires the TIMER2 to be

initialized once based on baud rate and data format, and bit 7 at I/O location 75 must be set. The utility called

AUTO485.exe is included in the /util directory.

If your project requires more than four serial ports, additional ports may be added via the PC/104 expansion bus.

Technologic Systems offers three different peripheral boards (TS-SER1, TS-SER2, and TS-SER4) that add 1,2,

or 4 extra COM ports respectively. Typically these would be configured as COM5 or be assigned other higher

COM I/O locations. Because DOS only directly supports four serial ports, any additional ports beyond four will

require software drivers if using DOS.

The TS-5700 PC/104 bus has IRQ 7, 9, 12 or 15 available for additional serial ports.

Note: IRQ7 is used by many PCMCIA cards.

Typically each serial port has a dedicated interrupt, but the TS-SER4 allows all four extra serial ports to share a

single interrupt. This is very helpful in systems with a large number of serial ports since there are a limited

number of IRQ lines available.

To configure COM3 and COM4 interrupts write to the Interrupt Configuration bits in I/O space at address 79h.

The reset condition has no interrupt configured for COM3 or COM4. See the table below for details.

Table 5.6 COM3 and COM4 Interrupt Configuration Register

Bit1

Bit0

COM3-COM4 Interrupt Configuration

No IRQ driven by COM3 or COM4 (default at reset)

COM3 drives IRQ5, COM4 drives IRQ6

COM3 and COM4 share IRQ5

COM3 and COM4 share IRQ6

Technologic Systems

at I/O 79h

http://embeddedARM.com/

5/2009

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