PAXUSB00 datasheet(27/32 Pages) REDLION

27 / 32 page

COMMUNICATION FORMAT

Data is transferred from the meter through a serial communication channel.

In serial communications, the voltage is switched between a high and low level

at a predetermined rate (baud rate) using ASCII encoding. The receiving device

reads the voltage levels at the same intervals and then translates the switched

levels back to a character.

The voltage level conventions depend on the interface standard. The table

lists the voltage levels for each standard.

Data is transmitted one byte at a time with a variable idle period between

characters (0 to ∞). Each ASCII character is “framed” with a beginning start bit,

an optional parity bit and one or more ending stop bits. The data format and

baud rate must match that of other equipment in order for communication to

take place. The figures list the data formats employed by the meter.

Start bit and Data bits

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

receiving device to prepare for reception of 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.

Parity bit

After the data bits, the parity bit is sent. The transmitter sets the parity bit to

a zero or a one, so that the total number of ones contained in the transmission

(including the parity bit) is either even or odd. This 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 ignored by the

receiving device. The PAX meter ignores the parity bit of incoming data and

sets the parity bit to odd, even or none (mark parity) for outgoing data.

Stop bit

The last character transmitted is the stop bit. 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. If 7 data bits and no parity is selected, then 2

stop bits are sent from the PAXI.

Character Frame Figure

LOGIC

RS232*

RS485*

INTERFACE STATE

TXD,RXD; -3 to -15 V

a-b < -200 mV

mark (idle)

TXD,RXD; +3 to +15 V

a-b > +200 mV

space (active)

* Voltage levels at the Receiver

COMMAND RESPONSE TIME

The meter can only receive data or transmit data at any one time (half-duplex

operation). During RS232 transmissions, the meter ignores commands while

transmitting data, but instead uses RXD as a busy signal. When sending

commands and data to the meter, a delay must be imposed before sending

another command. This allows enough time for the meter to process the

command and prepare for the next command.

At the start of the time interval t1, the computer program prints or writes the

string to the com port, thus initiating a transmission. During t1, the command

characters are under transmission and at the end of this period, the command

terminating character (*, $ or slave only <CR>) is received by the meter. The

time duration of t1 is dependent on the number of characters and baud rate of

the channel.

t1 = (10 times the # of characters) / baud rate

At the start of time interval t2, the meter starts the interpretation of the

command and when complete, performs the command function. This time

interval t2 varies from 2 msec to 15 msec. If no response from the meter is

expected, the meter is ready to accept another command.

If the meter is to reply with data, the time interval t2 is controlled by the use

of the command terminating character and the Serial Transmit Delay parameter

(dELAY). The ‘*’ or ‘<CR>’ terminating character results in a response time

window of the Serial Transmit Delay time (dELAY) plus 15 msec. maximum. The

dELAY

parameter should be programmed to a value that allows sufficient time for

the release of the sending driver on the RS485 bus. Terminating the command

line with “$” results in a response time window (t2) of 2 msec minimum and 15

msec maximum. The response time of this terminating character requires that

sending drivers release within 2 msec after the terminating character is

received.

At the beginning of time interval t3, the meter responds with the first

character of the reply. As with t1, the time duration of t3 is dependent on the

number of characters and baud rate of the channel. At the end of t3, the meter is

ready to receive the next command.

t3 = (10 times the # of characters) / baud rate

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

times t1, t2 and t3.

Ready

Command

String

Transmission

Unit

Response

Time

Command

Terminator

Received

NO REPLY FROM METER

Ready

Command

Terminator

Received

First

Character

of Reply

Transmission

Time

Unit

Response

Time

Command

String

Transmission

RESPONSE FROM METER

Timing Diagrams

PAXUSB00 Datasheet(PDF) 27 Page - Red Lion Controls. Inc

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