Red Lion PAXDP - Manual (Page 25)

25 25
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