Remote-processing RPC-330 Manuel d'utilisateur télécharger pdf (Page 16)

SERIAL PORTS SECTION 4

Page 4-3 RPC-330

Figure 4-3 Data packet

Two wire RS-485

The RS-485 port on the RPC-330 is set up for 4 wire

mode. 2- wire mode will cause the tra nsmitted data to

be received. To use the RPC-330 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-330 in a multidrop network by

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

32 units (including other RPC-330' 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-330, 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-330 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 interrupt

program uses a STR function to deter mine if the da ta

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

Line 20 sets up ON COM$ to interrupt on a < CR> and

branch to line 1000. Line 30 sets up this card' s address.

Line 1010 checks to see if the received message = this

card's address. If not, the subroutine ends. When there

is a match, further processing is performed.

ACCESSING SERIAL BUFFERS

1 2 ... 11 12 13 14 15 16 17 18 19 20 21 ... 47 48

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Remote-processing RPC-330 Manuel d'utilisateur Page 16

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