
9. MODBUS protocol and instructions
9.1 Communication connection
The RS-485 communication port of the transmitter features with electronic isolation protection,
lightning protection, and to provide internal independent ground solution. It is allowed to use
normal twisted-pair (segregation double-stranded twisted pair cable) cable connections. All devices
are in contact with a double-stranded, and then all together, and another line will be connected with
all the negative contacts, and the isolated shield wire must be connected to GND. When we talk
about communication in the laboratory, the stand-alone master-slave communication is relatively
simple. Hence, it is allowed to consider using the normal cable instead. However, there should be
strictly in accordance with the requirements of industrial engineering construction. Wiring diagram
is as follows:
Note:
1.
The RS-485 interface of PC-3110-RS transmitter has a protective earth terminal. When communicate with the
RS-485, there should use with solution ground to eliminate risk of safety.
2.
It is allowed to use an 120 ohm impedance matching resistors at terminal equipment in the transmission lines
(D +, D-) ends across to effectively reduce or eliminate signal reflection.
3.
Without repeaters, the RS-485 network can not exceed a maximum of 32 nodes. The maximum communication
transmission distance of RS-485 is up to 1200 meters. For long distance transmission, it is recommended to apply
cables which are dedicatedly design for RS-485.
4.
When communication, all the equipments of the network should be maintained in the same transfer mode, baud
rate, parity consistent. And each of the device address cannot be the same, so as not to conflict resulted in the
normal network communications.
5.
The Modbus command of the transmitter can only access 50 registers. If it exceeds the length, then it returns
abnormal message.
6.
The waiting time which a slave instrument response to a master machine is different according to each model.
Generally, it shall be longer than 0.5 second. (Some models may require a longer waiting-responding time,
please note whether the operation manual specified.)
V
Bus
GND
︴
D+(B)
GND Master
D-(A)
PC-3110-RS
ID: 01
PC-3110-RS
ID: 02
Other
Modbus
equipment
.....
︴
︴
Depending on
the actual
situation
Pull-up resistor
R
pull-high
Terminator
R
T
Pull-down
resistor
R
pull-low
53
Содержание PC-3110-RS
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