Remote-processing RPC-52 Manuel d'utilisateur télécharger pdf (Page 12)

SERIAL PORTS CHAPTER 4

Page 10

Figure 4-2 Network diagram

[1-3],[2-4] Termination network installed

[3-5],[4-6] Termination network removed

Only one slave device on a RS-485 network should have

a terminator installed. The host transmitter should also

have a 100 ohm resistor in series with a 0.1 mfd

capacitor. T he term inator on the RPC -52 includes pu ll

up and pull down resistors to prevent lines from floating

and generating er roneous char acters.

TWO WIRE RS-485

The RS-485 port on the RPC-52 is set up for 4 wire

mode. 2- wire mode w ill cause the tra nsmitted data to

be received. To use the RPC-52 is this mode, your code

should "flush" the received data or otherwise remove

transmitted information.

Mechanically, to make a 2- wire system, simply connect

T+ to R+ and T- to R -. M ake sure CON FIG BAUD is

set up for RS-485 mode.

MULTIDROP NETWORK

You can use the RPC-52 in a multidrop network by

using CO M1' s RS-422/ 485 port. You can c onnect up to

32 units (including other RPC-52' s) over a 4,000 foot

range.

Figure 4-2 shows an example of a multidrop network.

This network includes a host and one or m ore devices.

The host transmits data packets to all of the devices, or

nodes, in the network. A data packet includes an

address, com mand, data, and a checksum. See figure 4-

3. The packet is received by all devices, and ignored by

all except the one addressed.

The relationship described below between nodes and the

host is a master-slave. The host dir ects all

communication. Nodes "do not speak unless spoken to".

Peer to peer communication, while possible with the

RPC-52, is not discussed here.

There are m any communication protocols. F or this

example, a protocol might look som ething like this:

1 2 ... 7 8 9 10 11 12 13 14 15 16 17 ... 35 36

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Remote-processing RPC-52 Manuel d'utilisateur Page 12