Bulletin No. PAXCK-J
Released 2018-05-15
Drawing No. LP0524
-23-
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.
Character Frame Figure
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 PAX.
LOGIC
INTERFACE STATE
RS232*
RS485*
1
mark (idle)
TXD,RXD; -3 to -25 V
a-b < -200 mV
0
space (active)
TXD,RXD; +3 to +25 V
a-b > +200 mV
* 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.
Refer to the Timing Diagrams below. 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 (*, $) is received by the meter. The time duration of t
1
is
dependent on the number of characters and baud rate of the channel.
t
1
= (10 times the # of characters) / baud rate
At the start of time interval t
2
, the meter starts the interpretation of the
command and when complete, performs the command function. This
time interval t
2
varies. If no response from the meter is expected, the
meter is ready to accept another command.
If the meter is to reply with data, the time interval t
2
is controlled by the
use of the command terminating character. The '*' terminating character
results in a response time window of 50 msec. minimum and 100 msec.
maximum. This allows sufficient time for the release of the sending driver
on the RS485 bus. Terminating the command line with '$' 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. At the end of t
3
,
the meter is ready to receive the next command.
t
3
= (10 times the # of characters) / baud rate.
SERIAL
TIMING
Timing Diagrams
NO REPLY FROM METER
RESPONSE FROM METER
COMMAND
COMMENT
PROCESS TIME (t
2
)
R
Reset
2-50 msec.
V
Write
100-200 msec.
T
Transmit
2-50 msec. for $
50-100 msec. for *
P
2-50 msec. for $
50-100 msec. for *
Ready
Ready
1
t
t
2
Command
String
Transmission
Unit
Response
Time
Command
Terminator
Received
Ready
t
1
t
2
Ready
t
3
Command
Terminator
Received
First
Character
of Reply
Reply
Transmission
Time
Unit
Response
Time
Command
String
Transmission
