TS-7300 MANUAL
CONNECTORS AND HEADERS
Table: COM2 Serial Port Pin Configuration of Connectors
COM2
Header
DB9
Connector
RS-232
Half Duplex
RS-485
Full Duplex
RS-485
1
1
TX+ / RX+
TX+
2
2
RXD
3
3
TXD
4
7
RX+
5
5
GND
GND
GND
6
6
TX- / RX-
TX-
7
7
8
8
9
9
RX-
10
-
-
-
-
Adapter cables are available to convert COM2 to a standard 9-pin SubD male connector.
In addition, COM2 has optional support for full or half-duplex RS-485
Note
The COM2 serial port 10-pin header has both RS-232 and RS-485 signals. Only
one set may be used for a given application – it is not possible to use both at the
same time.
Optional RS-485 Support
The COM2 Port supports RS-485 half duplex or full duplex operation. RS-485 drivers
allow communications between multiple nodes up to 4000 feet (1200 meters) via twisted
pair cable. Half-duplex RS-485 requires one twisted pair plus a Ground connection, while
full duplex requires two twisted pair plus a Ground. For half-duplex operation, a single
twisted pair is used for transmitting and receiving.
The ioctl Linux system call is used 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. 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. In a typical RS-485
installation, the RS-485 drivers are frequently “tri-stated.” If fail-safe bias resistors are not
present, the RS-485 bus may “float” and small amounts of noise can cause spurious
characters at the receivers. 4.7K Ohm 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.
Note
A special ioctl command has been added to the TS-Kernel's serial code to turn off
and on the automatic RS-485 feature. Refer to the Linux for TS-ARM User's Guide
for additional software information on RS-485 support.
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