Technologic Systems TS-7200, Руководство пользователя, Страница: 13 / 52

For half-duplex operation, a single twisted pair is used for transmitting and receiving.
The Auto485.exe utility must be run to initialize the automatic RS-485 control logic. For
full-duplex operation, two twisted pairs are used and the transmit driver is typically
enabled all the time. The auto485.exe utility must be run to put the logic into full-duplex
RS-485 mode.
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 RS-485 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. 4.7KW 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 in many installations are not required when cable lengths are less
than 50 meters and the baud rate is 9600 or less. Termination resistors may be required
for reliable operation when running longer distances or at higher baud rates.
Termination resistors (typically 100-150 ohms) 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 0x2240_0000 bit 1 will return a "1" when the RS-485 option is
installed.
3.4 RS-485 Quick-Start Procedure
Perform the following steps:
1. Check that the RS-485 option is installed in the TS-7200 (required).
2. Attach the RS-485 cable to the 10-pin header. (Adaptor cables to SubDB9
connector are available from Technologic Systems.)
3. Set the COM2 UART serial parameters (baud rate, data, parity, and stop bits,
interrupts, etc) in your source code
4. Use the TIOC_SBCS485 ioctl() to enable the automatic RS-485 feature.
5. Simply read () and write () data to the COM2 UART – the automatic RS-485 logic
controls TX enable.
TS-7200 User's Manual Technologic Systems http://www.embeddedARM.com/
9 12/2004