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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.
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
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
The maximum serial throughput of the meter is limited to the sum of the
times t 1 , t 2 and t 3 .
NO REPLY FROM METER
characters are under transmission and at the end of this period, the command
terminating character (* or $) is received by the meter. The time duration of t 1
is dependent on the number of characters and baud rate of the channel.
Command
String
Transmission
Meter
Response
Time
t 1 = (10 times the # of characters) / baud rate
Ready
t 1
t 2
Ready
At the start of time interval t 2 , the meter starts the interpretation of the
command and when complete, performs the command function. This time
interval t 2 varies. If no response from the meter is expected, the meter is ready
to accept another command.
If the meter is to reply with data, the time interval t 2 is controlled by the use
of the command terminating character. The ‘*’ terminating character results in
RESPONSE FROM METER
a response time of 50 msec. minimum. This allows sufficient time for the
Ready
t 1
t 2
t 3
Ready
Timing Diagram Figure
release of the sending driver on the RS485 bus. Terminating the command line
with ‘$’ results in a response time (t 2 ) of 2 msec. minimum. The faster response
time of this terminating character requires that sending drivers release within 2
msec. after the terminating character is received.
Command First
Terminator Character
Received of Reply
Reply
Transmission
Time
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.
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
LOGIC
1
INTERFACE STATE
mark (idle)
RS232*
TXD,RXD; -3 to -15 V
RS485*
a-b < -200 mV
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
0 space (active) TXD,RXD; +3 to +15 V
* Voltage levels at the Receiver
a-b > +200 mV
(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
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.
number of bits. Given this limitation, the parity bit is often ignored by the
receiving device. The LD Timer 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 meter.
Character Frame Figure
13
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