Remote-processing RPC-30 Manual do Utilizador descarregar pdf (Página 15)

CHAPTER 4 SERIAL PORTS

RPC-30 Page 13

Figure 4-2 Network diagram

Only one 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 -30 includes pu ll

up and pull down resistors to prevent lines from floating

and generating er roneous char acters.

MULTIDROP NETWORK

You can use the RPC-30 in a multidrop network by

using CO M2' s RS-422/ 485 port. You can connect up to

32 units (including other RPC-30' 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-30, is not discussed here. The problem is defining

a bus arbitration scheme, of which there are many.

There are m any communication protocols. F or this

example, a protocol might look som ething like this:

> 22MB1

The protocol starts w ith the < cr> character. This

character synchronizes all units and alerts them that the

next few characters coming down are address and data.

In this case, "> 22" is the units address. "M " is the

comm and and " B1" is the checksum . T he comm and is

terminated with a < cr> character.

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