TS-2800 User’s Manual Technologic Systems
05/21/09
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5.3 RS-485 Support
An option is available to add support to COM1 for half
duplex RS-485. 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.
For half-duplex operation, a single twisted pair is used
for transmitting and receiving. The serial port's RTS
signal controls the RS-485 transmitter/receiver. When
RTS is asserted true (bit 1 of the modem control register
= 1), the RS-485 transmitter is enabled and the receiver disabled. When RTS is de-asserted the
transmitter is tri-stated (disabled) and the receiver is enabled. Since the transmitter and receiver are
never both enabled, the serial port UART does not receive the data transmitted. The transmitter and
receiver share a single pair of signals that are available in a screw-down terminal block. See the table
below for pin-outs Note: the correct jumper must be installed. See the next table for details.
Fail-safe bias resistors are used to bias both the X- and X+ 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 will be floating and very small amounts of noise
can cause spurious characters at the receivers. A 1.2KΩ resistor is
used to pull the X+ signal to 5V and another 1.2KΩ resistor pulls the X- signal to ground. Termination
resistors are also required for reliable operation particularly when running long distances. 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.
Installing JP6 adds a 100Ω termination resistor to the
RS-485 line.
When the RS-485 option is installed, jumper JP7
enables the RS-485 receiver to drive the COM1 UART.
When JP7 is not installed, the RS-232 receiver drives
the COM1 UART.
A read at I/O location 76h bit 0 will return a "1" when the
RS-485 option is installed.
5.4 Adding Serial Ports
If your project requires more than two serial ports, additional ports may be added via the PC/104
expansion bus. Technologic Systems currently offers a 2 serial / 1 parallel port card, and other
manufacturers sell cards with up to four additional serial ports. Typically these would be configured as
COM3 or COM4 or be assigned other non-standard I/O locations. Because DOS only directly supports
four serial ports, any additional ports beyond four will require software drivers.
The PC/104 bus has IRQ3, 4, 5, 6, 7 or 9 available for additional serial ports. If IRQ3 or 4 are to be
used on a PC/104 expansion card, then care must be taken since COM2 and COM1 also use these
IRQs, respectively. For example, if IRQ4 is used for COM3 then either COM1 must be used in a non-
interrupt fashion or only one COM port can have the interrupt enabled at a time. In any case only one
RS-485 Quick start procedure:
1. The RS-485 option must be installed
2. Install JP7 to enable RS-485 operation
3. Attach the RS-485 cable to the 3-pin
terminal strip connector.
4. Set the COM1 UART serial
parameters (baud rate, data, parity,
and stop bits, interrupts, etc).
5. To transmit data, assert RTS and write
the data to the UART
6. To receive data, deassert RTS and
read the data from the UART
Position (with terminal
strip facing you) Left Center Right
Silk-screen Label X+ X- GND
Half-duplex Usage TX+ / RX+ TX- / RX- GND
Table 1 - RS-485 Signals
I/O
Address R / W Bit 0 Description
76h Read RS-485 option
0 = Not installed
1 = Option installed
77h Read Returns status of TX485EN pin
Table 3 – TS-2800 Control Registers – All signals are
read only through bit 0 of the address
JP7 Receiver Source
Installed Half-duplex RS-485
Not
installed RS-232
Table 2 - COM1 Receiver Source
