Ampro Corporation 486E - Manual (Page 33)

Product Reference
2–17
Using the RS-485 Interface
The RS-485 interface allows half-duplex operation using a 5 VDC differential interface. This
interface provides greater immunity against noise and interference than single-wire interfaces and
can drive cable lengths up to 4000 feet reliably at 57.6K bps. All communication, both transmission
and reception, occurs through a single pair of wires. There are no handshaking lines.
RS-485 supports multidrop operation, where more than two devices can be connected to the same
RS-485 balanced line. To prevent signal contention, only one transmitter is enabled at a time. The
Little Board/486e RS-485 transmitter is controlled by Serial 2’s RTS signal. At power up, RTS is in
its inactive state, ready to receive. When it is time to transmit, the RTS signal is made active,
enabling the transmitter. It is the responsibility of the user’s software to prevent two transmitters
from being enabled at the same time.
Figure 2–7 illustrates the Little Board/486e RS-485 interface wiring.
Figure 2–7. RS-485 Interface Wiring
Interconnection Scheme Examples
The following interconnection scheme examples take advantage of the RS-485 serial connection:
One-way Broadcast
A single device uses an RS-485 signal pair to transmit data to many receiving devices. When the
RTS signal is turned on (True, High, and Active) and left on, the broadcast transmitter is enabled.
If the device is to be a receiver, RTS must be turned off and left off.
Simple Bi-Directional Communication
Two devices using a single RS-485 bi-directional pair for half-duplex provide two-way transmission
of data. The transceiver is placed in the send or receive mode under control of the network software
using either a simple alternation scheme or by messages contained within data packets.
Serial 2 RxD
W5 RS-485
RxD
Receiver
W6
Transmitter
Serial 2 TxD
Serial 2 RTS
Transmitter Enable
RS-232 RxD
J6-1 +I/O
J6-2 -I/O
100
Ohms