D-MAX DMC-20SEC Manual de usuario descargar pdf (Pagina 15)

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

Macro Sensors Tel. (856) 662-8000

www.macrosensors.com

Serial Communications

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TXD, RXD: -3 to -15 V

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Mark (idle)1

TXD, RXD: +3 to +15 V

Logic

Interface State

RS-232

Communication Formats

ASCII Mode

The ASCII mode is a simple, isolated ASCII communication

protocol using the standard ASCII character set. This mode

provides external communication between the meter and a PC

allowing remote programming to be carried out.

DMC-A2 Series meters use a serial communication channel to

transfer data from the meter to another device. With serial

communications, data is sent one bit at a time over a single

communications line. The voltage is switched between a high

and a low level at a predetermined transmission speed (baud

rate) using ASCII encoding. Each ASCII character is transmitted

individually as a byte of information (eight bits) with a variable

idle period between characters. The idle period is the time

between the receiving device receiving the stop bit of the last

byte sent and the start bit of the next byte. The receiving device

(for example a PC) reads the voltage levels at the same interval

and then translates the switched levels back to an ASCII

character. The voltage levels depend on the interface standard

being used.

Table 1 lists the voltage level conventions used for RS-232. The

voltage levels listed are at the receiver.

See Table 2 for a list of the most commonly accessed ASCII

mode registers.

Each ASCII character is 'framed' with:

• A start bit.

• An optional error detection parity bit.

• And one or more ending stop bits.

For communication to take place, the data format and baud rate

(transmission speed) must match that of the other equipment in

the communication circuit. Figure 5 shows the character frame

formats used by the meter.

Idle

8 data, no parity, 1 stop

Idle

8 data, parity, 1 stop

Note: b

to b

is ASCII data.

Start

Bit

Stop

Bit

Receiving Device (PC)

Sending Device (Meter)

Data Bits

Stop

Bit

Parity

Bit

Figure 5 – Character Frame Formats

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Function

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1 sutatSmralA 00004

2 retsigeRyalpsiD 31504

3 tluseRataDdess

ecorP 51504

4 1lennahC–ataDdessecorP 71504

5 2lennahC–ataDdessecorP 91504

93 3lennahC–ataDdessecorP 12504

04 4len

nahC–ataDdessecorP 32504

6 1tniopteS 53504

7 2tniopteS 73504

8 3tniopteS 93504

9 4tniopteS 14504

01 5tniopteS 34504

6tniopteS 54504

21 kaeP 52504

31 yellaV 72504

41 eraT 33504

51 esUetamxeTrofdevreseR –

61 1latoT 92504

71 2latoT 1350

ASCII Reg. #

Character Frame Formats

Start Bit and Data Bits

Data transmission always begins with the start bit. The start bit

signals the receiving device to prepare to receive data. One bit

period later, the least significant bit of the ASCII encoded

character is transmitted, followed by the remaining data bits.

The receiving device then reads each bit position as they are

transmitted and, since the sending and receiving devices

operate at the same transmission speed (baud rate), the data

is read without timing errors.

Parity Bit

To prevent errors in communication, the sum of data bits in

each character (byte) must be the same: either an odd amount

or an even amount. The parity bit is used to maintain this

similarity for all characters throughout the transmission.

It is necessary for the parity protocol of the sending and

receiving devices to be set before transmission.There are three

options for the parity bit, it can be set to either:

• None – which means there is no parity.

• Odd – which means the sum of bits in each byte is odd.

• Even – which means the sum of bits in each byte is even.

After the start and data bits of the byte have been sent, the

parity bit is sent. The transmitter sets the parity bit to 1 or 0

making the sum of the bits of the first character odd or even,

depending on the parity protocol set for the sending and

receiving devices.

As each subsequent character in the transmission is sent, the

transmitter sets the parity bit to a 1 or a 0 so that the protocol of

each character is the same as the first character: odd or even.

The parity bit is used by the receiver to detect errors that may

occur to an odd number of bits in the transmission. However, a

single parity bit cannot detect errors that may occur to an even

number of bits. Given this limitation, the parity bit is often

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