Remote-processing RPC-2350 Manuel d'utilisateur télécharger pdf (Page 21)

SERIAL PORTS CHAPTER 4

4-2

Comm unication flow control

There are two methods to control transmission:

Hardware and software. Both are discussed below.

When RTS line in goes low, the RPC-2350 is held off

from transmitting out CO M2. The status of this port is

read by the BIT statement. The example below returns

the status of the RTS line:

100 B = BIT(130,5)

If B = 1, transmission is held off.

You should check this line before executing a PRINT #2

statement. If transmission is held off, the print buffer

can becom e full. PRIN T #2 com mand w ill ‘hang’ until

the buffer accepts all characters in the program line.

This may give the appearance of the program locking up

or running very slowly. This could also result in a

“deadly embrace” situation wher e the receiving device is

expecting some other condition before it will receive

characters.

The CTS line may be set high or low to control

comm unication fr om a device when using RS-232 . T his

line is also used in RS-422/485 comm unication at P3 or

J3 to control transmitter output.

When using RS-232 comm unications, you may have to

initialize the CT S output to high to allow the sender to

transmit. The examples below show how this is done.

Line 400 sets CTS low and 500 sets it high.

400 BIT 128,4,1

500 BIT 128,4,0

A low condition is supposed to hold off a device. Some

devices do not recognize CTS output so this may be a

mute point.

CTS is used for RS-422/485 communications to turn

on/off the transm itter. Use C ONF IG BAU D to not only

set the baud rate but also the type of communication

protocol. CAMBASIC takes care of the CTS line for

RS-422 and RS-485.

The CONFIG BAUD statement sets both serial ports for

baud rate and type (RS-232, RS-422, and RS-485) for

COM 2.

XON/XOFF

The so-called XON/XOFF protocol is a software scheme

to enable and disable transmitting. Sending an XOFF

character (ASCII value 13H or < Ctrl-S> ) to the RPC-

2350 will stop transmission until an XON (ASCII value

11H or < Ctrl-Q> ) is received. This does not over-ride

the RTS input line when RTS is low.

RS-232

The most common communication interface is RS-232.

Quite simply, RS-232 defines only ‘0' and ‘1' voltage

levels and transient speeds. It does not define baud rate,

start and stop bit length, parity or the protocol. RS-232

is available at J1(COM1) and J4 (COM 2).

Usually, when people speak of RS-232, they mean

something that is compatible with a PC. However, you

must make sure the baud rate, parity, and stop bits are

proper for your device. These are set in the CONFIG

BAUD com mand.

RS-232 is used in point-to-point communication. That

is, one device talks to one other device. Schemes have

been devised to network RS-232.

The maximum baud rate RS-232 supports depends upon

the distance and cable characteristics. As a rule of

thumb, 9600 baud w ill work r eliably to 50 feet.

RS-422

RS-422 is used for long distance (1000 meter s)

comm unication. It is similar to R S-232 in that it is

point-to-point and the communication “protocol” is the

same.

The RS-485 port is used for RS-422 communication.

The transmitter is always on. If, for some reason, you

want to shut the transmitter off, execute

BIT 128,4,0

in your code. To turn the transm itter back on, execute

BIT 128,4,1

in your code.

RS-485

RS-485 is a popular networking system. Technically,

RS-485 only defines electrical specifications, not

protocols. (See Multi-drop Network below. ) In RS-

485, the transm itter is turne d on only long e nough to

send a message, then it turns off. Thus, RS-485 allows

multiple units to be put on a single set of wires.

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Remote-processing RPC-2350 Manuel d'utilisateur Page 21

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