26

COMMAND RESPONSE TIME

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

operation). 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 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

(). The standard command line terminating character is '*'. This

terminating character results in a response time window of the Serial Transmit

Delay time () plus 15msec. maximum. The  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.

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.

NO REPLY FROM METER

RESPONSE FROM METER

Timing Diagrams

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 PAXDP.

Character Frame Figure

LOGIC

RS232*

RS485*

INTERFACE STATE

1

TXD,RXD; -3 to -15 V

a-b < -200 mV

mark (idle)

0

TXD,RXD; +3 to +15 V

a-b > +200 mV

space (active)

* Voltage levels at the Receiver