Remote-processing RPC-52 Manuel d'utilisateur télécharger pdf (Page 13)

CHAPTER 4 SERIAL PORTS

Page 11

Figure 4-3 Data packet

> 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.

A response depends upon the nature of the command.

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

status". The RPC-52 could read the port and respond

with AA2< cr> . The first A is an acknow ledge, 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.

ACCESSING SERIAL BUFFERS

You can access C OM0 and COM 1 buffers in three w ays:

1. INPUT statement. This removes all characters in

the buffer up to the term inator cha racter and puts

them into a variable.

When using the INPUT statement, program

execution is suspended until a < cr> (Enter key) is

received. W hether this is a problem depends on

your particular application.

INPUT strips bit 7. This means ASCII characters

from 0 to 127 are received. The IN PUT statement

can return a maximum string length of about 150

characters.

2. GET function. Characters ar e rem oved one at a

time as a numerical value. A 0 is returned when the

buffer is empty. Use the C OM function to

determ ine if the buffer is empty or if a 0 is a data

value.

If you don' t read the buffer an d the buffer fills, all

subsequent characters are discarded. GET m ay be

used anywhere in the program.

3. COM$(n) retrieves all characters in the buffer,

including other control codes (except CR).

ACCESSING COM0 AND COM1

The port INPUT and GE T functions retrieve data using

the UIn comm and. UI0 routes inputs to C OM0 while

UI1 routes inputs to the COM1 port. PRINT outputs are

set by the UOn com mand. UO0 p rints out COM0 while

UO1 outputs COM 1.

The following show how UIn and UOn work.

100 UI0 Set to COM0

110 INPUT A Get data from COM0 port

520 UI1 Switch to COM1 port

530 INPUT B Get data from COM1 port

800 UO0 Print to COM0

810 PRINT "Temperature:",T

900 UO1 Print to COM1

910 PRINT "Set pressure at:",CA

Power up default is set to COM0. UIn has no effect

upon COM $(n) function.

1 2 ... 8 9 10 11 12 13 14 15 16 17 18 ... 35 36

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Remote-processing RPC-52 Manuel d'utilisateur Page 13

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