KROHNE MFC 010 C Converter - Manual (Page 23)

MFC010 Interface Manual 21
Single 16 Bit Register Variables, Data Transmission Order
Byte 1, MS Byte Byte 0, LS Byte
Long Integer & Floating Point Variables, Data Transmission Order
Byte 1 Byte 0, LS Byte Byte 3, MS Byte Byte 2
Requested Register Requested Register + 1
Double Precision Floating Point Variables, Data Transmission Order
1 0 3 2 5 4 7 6
Requested Register Register + 1 Register + 2 Register + 3
5.4 Multidrop Operation
A “Master” dev ice, s uch as a P C or P LC, c an be us ed t o control and i nterrogate a num ber of “ Slave”
devices, such as an MFC010, connected to the Modbus bus in a “Multidrop” configuration. The “Master”
device always initiates the communication interchange with the “Slave” devices, each of which waits for
instructions or requests from the “Master” before transmitting dat a on t he bus i n r esponse t o t he
instruction. Although the Modbus specification allows for up to 247 “Slave” devices to be physically
connected to the bus at any time, the Master device can only request information from one “Slave” device
at a time. A unique ID number or “Address” is allocated to each of the “Slave” devices to allow the
“Master” to differentiate between them. Although it does not matter in which order the “Slave” devices are
interrogated, the “Master” must wait for the response, or for a suitable period after the request, before
making a request to any other of the slave devices on the bus.
Under some limited conditions, i.e. when the instruction to the “Slave” device does not require a detailed
response, the “Master” device can send a “Broadcast” command, indicated by a “Slave” ID Address of “0”,
to all of the slave devices simultaneously.
5.5 Calculating Data Transmission Rates
Careful attention should be made to ensuring that the bus installation can support the amount and rate of
data t ransmission r equired. C onsideration of t he l imitations of the physical installation, as previously
described, s hould not be i gnored. T he m aximum us able t ransmission s peed, baud r ate, w ill depend
entirely upon the installation.
The transmission format also needs to be carefully considered. In the Modbus standard, each transmitted
character is 11 bi ts long, depending upon t he setting of the transmission format. A t the Modbus default
transmission speed of 19200 baud, each character will have a transmission period of 573 microseconds.
For a simple data transfer of one Input value (see section 6.4 on page 26 for details) between the master
and slave will require an 8 character (+ 2 x 3½ character “Quiet” periods) telegram in the request from the
master, and a 9 character (+ 2 x 3½ character “Quiet” periods) telegram in the response from the slave. If
the Slave responds immediately the cycle from the Master sending the request to receiving the response
will be at least 31 characters long, or 17.8 milliseconds at 19200 baud. T herefore the maximum rate of
data requests that could be made is 56 every second.
In m ost c ases f ar m ore dat a w ill be required, and i n m ultidrop s ystems t he m aster dev ice m ay be
requesting data from up to 64 units. In these circumstances the user must ensure that there is sufficient
time i nterval bet ween r equests f or t he m easured v alues t o be received without overlapping the Master
request telegram with the previous slave reply telegram.
To achieve the required update rates the user may have to consider whether, in a multidrop configuration,
the number of devices on a bus must be l imited or whether the cable installation will support one of the
higher data transmission speeds which are available.
This is especially important where fast response is required (such as batch filling operations).