29
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 t
1
, the computer program prints or writes the
string to the com port, thus initiating a transmission. During t
1
, 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 t
1
is
dependent on the number of characters and baud rate of the channel.
t
1
= (10 * # of characters) / baud rate
At the start of time interval t
2
, the meter starts the interpretation of the
command and when complete, performs the command function. This time
interval t
2
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 t
2
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 (t
2
) 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 t
3
, the meter responds with the first
character of the reply. As with t
1
, the time duration of t
3
is dependent on the
number of characters and baud rate of the channel. t
3
= (10 * # of characters) /
baud rate. At the end of t
3
, the meter is ready to receive the next command.
The maximum serial throughput of the meter is limited to the sum of the
times t
1
, t
2
and t
3
.
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*
1 mark (idle) TXD,RXD; -3 to -15 V a-b < -200 mV
0 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 Ready
1
t t
2
Ready
t
1
t
2
Ready
t
3
Command
String
Transmission
Meter
Response
Time
Command
Terminator
Received
First
Character
of Reply
Reply
Transmission
NO REPLY FROM METER
RESPONSE FROM METER
Time
Timing Diagram Figure
IDLE 0
b
01
b
2
bb
3
b
4
b
56
b
1 IDLE
IDLEIDLE 0
bb
0
b
132
bb
4
b
5
b
6
P1
IDLEIDLE 0
bbb
01
bb
324
bb
56
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.
0
7
7
b
Character Frame Figure
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