Ampro Corporation 486E manual Using the RS-485 Interface, Interconnection Scheme Examples

Product Reference

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.

Serial 2 TxD

Serial 2 RTS Transmitter Enable

Serial 2 RxD

RS-232 RxD

Transmitter

J6-1 +I/O

J6-2 -I/O

100

Ohms

W5	RS-485	W6
	RxD

Receiver

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.

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