D-MAX DMC-20SEC Manual de usuario descargar pdf (Pagina 16)

Macro Sensors Tel. (856) 662-8000

11 May, 2006 DMC-A2 Series (ALD-00029)

www.macrosensors.com

Serial Communications (cont.)

ignored by the receiving device. The user sets the parity bit of

incoming data and sets the parity bit to odd, even or none (mark

parity) for outgoing data.

The parity bit is set in the Calibration Mode.

Stop Bit

The stop bit is the last character to be transmitted. The stop bit

provides a single bit period pause to allow the receiver to

prepare to re-synchronize to the start of a new transmission

(start bit of next byte). The receiver then continuously looks for

the occurrence of the start bit.

Note: DMC-A2 Series

meters use only one stop bit.

Command Response Time

The meter uses half-duplex operation to send and receive data.

This means that it can only send or receive data at any given

time. It cannot do both simultaneously. The meter ignores

commands while transmitting data, using RXD as a busy signal.

When the meter receives commands and data, after the first

command string has been received, timing restrictions are

imposed on subsequent commands. This allows enough time

for the meter to process the command and prepare for the next

command.

See Figure 6. At the start of the time interval t

, the sending

device (PC) prints or writes the string to the com port, thus

initiating a transmission. During t

the command characters are

under transmission and at the end of this period the command

terminating character is received by the meter. The time

duration of time interval t

is dependent on the number of

characters and baud rate of the channel:

= (10 * # of characters) / baud rate

At the start of time interval t

, the meter starts to interpret the

command, and when complete performs the command

function.

After receiving a valid command string, the meter always

indicates to the sending device when it is ready to accept a new

command. After a read command, the meter responds with the

requested data followed by a carriage return (øDH) and a line

feed (øAH) character. After receiving a write command, the

meter executes the write command and then responds with a

carriage return/line feed.

The sending device should wait for the carriage return/line feed

characters before sending the next command to the meter.

If the meter is to reply with data, time interval t

is controlled by

using the command terminating character: $ or *. The $

terminating character results in a response time window of 50

ms minimum and 100 ms maximum.This allows enough time to

release the sending driver on the RS-485 bus. Terminating the

command line with the * symbol, results in a response time

window (t

) of 2 ms minimum and 50 ms maximum. The faster

response time of this terminating character requires that

sending drivers release within 2 ms after the terminating

character is received.

At the start of time interval t

, the meter responds with the first

character of the reply. As with t

, the time duration of t

dependent on the number of characters and baud rate of the

channel:

= (10 * # of characters) / baud rate

At the end of t

the meter is ready to receive the next command.

The maximum throughput of the meter is limited to the sum of

the times: t

, t

Ready

Response From The Meter

Command

Terminator

Received

First

Character

of Reply

Ready

Transmission

Time

Figure 6 – Timing Diagram

ASCII Serial Mode Read/ Write

Information

ASCII Command Character Descriptions

Table 3 describes the functions of the command string

characters.Table 4 shows examples of how the command string

is constructed.

Command String Construction

When sending commands to the meter using a Terminal

emulation program, a string containing at least one command

character must be constructed. A command string consists of

the following characters and must be constructed in the order

shown:

1) A start character.

2) The meter (node) address (optional).

3) The read/write command.

4) The register address.

5) A separator character.

6) The data value.

7) The message terminator.

Figure 7 shows an example of a command string.

Start

Character

Meter

Address

Read/Write

Command

Address

Separator

Character

Data

Value

Message

Terminator

Sr130*

Figure 7 – Example of a Command String

1 2 ... 11 12 13 14 15 16 17 18 19 20 21 ... 27 28

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