CBLUSB01 datasheet(29/36 Pages) REDLION

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Command Response Time

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

operation). 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 (*) 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 * # 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 50 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. The standard command line terminating

character is ‘*’. This terminating character results in a response time window of

50 msec minimum and 100 msec maximum. This 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 50 msec

maximum. The faster 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. t3 = (10 * # of characters) /

baud rate. At the end of t3, the meter is ready to receive the next command.

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

times t1, t2 and t3.

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.

LOGIC

INTERFACE STATE

RS232*

RS485*

mark (idle)

TXD,RXD; -3 to -15 V

a-b < -200 mV

space (active)

TXD,RXD; +3 to +15 V

a-b > +200 mV

* Voltage levels at the Receiver

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 error detection 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. Since the sending and receiving devices operate at the same

transmission speed (baud rate), the data is read without timing errors.

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.

Ready

Command

String

Transmission

Meter

Response

Time

Command

Terminator

Received

First

Character

of Reply

Transmission

NO REPLY FROM METER

RESPONSE FROM METER

Time

Timing Diagram Figure

IDLE 0 b01

bb3 b4 b56

1 IDLE

IDLE

IDLE 0

bb4 b5 b6 P1

IDLE

IDLE 0

b56 11

(7 data, no parity, 2 stop)

(7 data, parity, 1 stop)

(8 data, no parity, 1 stop)

Start bit

Stop bit

Note: b - b is ASCII data.

Character Frame Figure

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