TS-5500 User’s Manual
Technologic Systems
10/31/03
9
Note:
the correct jumper (FD or HD) must be installed. See the Table 2 for details
.
Fail-safe bias resistors are used to bias the TX+, TX- and RX+,
RX- lines to the correct state when these lines are not being
actively driven. This is an important consideration, since in a
typical RS-485 installation, the drivers are frequently tri-stated. If
fail-safe bias resistors are not present, the 485 bus may be
floating and very small amounts of noise can cause spurious
characters at the receivers. 4.7K
Ω
resistors are used to pull the
TX+ and RX+ signals to 5V and also to bias the TX- and RX-
signals to ground. Termination resistors may be required for reliable operation when running long
distances at high baud rates. Termination resistors should only be installed at each end of an RS-485
transmission line. In a multi-drop application where there are several drivers and/or receivers attached,
only the devices at each end of the transmission line pair should have termination resistors.
A read at I/O location 75h bit 1 will return a "1" when the RS-485 option is installed.
5.4 Automatic RS-485 TX Enable
TS-5500 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
COM3 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 COM3 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. A utility called AUTO485.exe is included in the AUTOEXEC.bat that simplifies this task.
5.5 Configuring COM3
COM3 has a header labeled HD3 with 6 jumper positions. Three of these jumpers select which interrupt
COM3 uses (IRQ5, IRQ6 or IRQ7). Only one of the interrupt jumpers should be installed. .
Note:
IRQ7 is used by many PCMCIA cards. The other three jumper positions select RS-232, RS-485 (Half-
Duplex), or RS-422 (Full-Duplex) operation. Only one of these jumpers should be installed.
Linux strongly prefers the use of interrupts on all COM ports. If COM3 does not have a jumper installed
for interrupt selection, extremely slow throughput is a common symptom under Linux. IRQ5 is the most
commonly used IRQ for COM3. Regardless of IRQ selected, run the setserial command with the
following parameters to register the interrupt with the kernel:
setserial -v /dev/ttyS2 auto_irq autoconfig
5.6 Adding Serial Ports
If your project requires more than three serial ports, additional ports may be added via the PC/104
expansion bus. Technologic Systems offers three different daughter boards (TS-SER1, TS-SER2, and
TS-SER4) that add 1,2,or 4 extra COM ports respectively. Typically these would be configured as
COM4 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-5500 PC/104 bus has IRQ 5, 6, 7, 12 or 15 available for additional serial ports.
Note:
IRQ7 is used by many PCMCIA cards and COM3 will typically use IRQ5 or IRQ6.
Jumper
Receiver Source
FD
Full-Duplex RS-485
HD
Half-Duplex RS-485
232
RS-232
Table 1 – COM3 Receiver Source
Содержание TS-5500
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