Remote-processing RPC-210 Manual de usuario descargar pdf (Pagina 22)

SERIAL & SPI PORTS BASIC SECTION 4

Page 4-4

Figure 4-4 Data packet

RS-422/485 Termination network

W10 enables the network terminator. When the RPC-

210 is the last physical unit on a network (RS-485) or is

the only unit (RS-422), the receiver must be terminated

to prevent ringing and noise pick up. Insert jum pers into

W10[1-2][3-4]. See Figure 4-2 above for jumper

example.

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

a terminator installed. The host transmitter should also

have a 120 ohm resistor in series with a 0.01 mfd

capacitor. T he term inator on the RPC -210 includes p ull

up and pull down resistors to prevent lines form floating

and generating er roneous char acters.

RS-485 transmitter turn-off

The RS-485 transmitter is controlled by the RPBASIC-

52 operating system. The 485 transmitter is turned off

when the last character is sent. You must specify RS-

485 mode in the CONFIG BAUD 1 comm and.

There is a small potential for having two transmitters on

at the same time. The RS-485 transmitter on the RPC-

210 stays on for approximately ½ character time after

the last byte is sent. Nor mally, this is not a problem

when baud rates are 9600 and above as receivers usually

do not respond with 1 mSec anyway. However, if you

are transmitting at 4800 baud and below, there is this

potential. When a host (or other peer) is responding,

ensure there is a character time delay before responding.

Multidrop Network

You can use the RPC-210 in a multidrop network by

using COM1's RS-422/485 port or even a radio modem.

You can connect up to 32 units (including other RPC-

210' s) over a 4 ,00 0 foot range. The num ber of r adio

modems is virtually unlimited.

A network includes a host and one or more 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-

2. 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-210, is not discussed here.

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.

The response depends upon the nature of the command.

Suppose the command M means "return a digital I/O

port status". The RPC-210 could read the port and

respond with AA2< cr> . The first A is an

acknowledge, that is no errors were detected in the

message. The data, A2, can be broken down as follows:

Bit/line 7 6 5 4 3 2 1 0

Status 1 0 1 0 0 0 1 0 = A2

Lines 1, 5 and 7 are high while the others are low.

The following program fragment uses ON C OM$ and

STR in a network environment. ON COM$ generates an

interrupt when a < CR> is received. The interr upt

program uses a STR fun ction to deter mine if the data

packet was addressed to this card.

10 STRING 200,20

20 ON COM$ 1,0,13,1000

30 $(1) = ">05"

1000 $(0) = COM$(1)

1010 A = STR(8,$(0),$(1))

1020 IF A = 0 THEN RETURN

1 2 ... 17 18 19 20 21 22 23 24 25 26 27 ... 59 60

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Remote-processing RPC-210 Manual de usuario Pagina 22

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