HD45 / HD46
- 41 -
V2.4
•
Characteristic impedance: 120 ohm
•
Capacity: less than 50pF/m
•
Resistance: less than 100 ohm/km
•
gauge: 0,22 mm
2
(AWG24) at least
If more than 32 devices have to be connected (that is, 31 instruments plus the Master
unit), place a signal repeater between a group and the next one. The line termination
must be connected at both ends of each segment.
The cable maximum length depends on baud rate and cable characteristics. Typically,
the maximum length is 1200m.
The data line must be kept separated from any power lines in order to prevent
interferences on the transmitted signal.
For connection to a PC, a RS232/RS485 or a USB/RS485 converter must be used.
Each transmitter placed on the network is univocally identified by an address, whose
value ranges within 1 and 247.
Transmitters having the same address shall not
be present in the network,
otherwise the “Master” unit will not be able to
communicate with these devices due to signal conflicts in the line.
The default communication parameters are as follows:
•
Baud rate: 19200
•
Parity: Even
•
Start bit:
1
•
Data bits: 8
•
Stop bit:
1
Transmitter address, baud rate, parity type and stop bits number are configurable
parameters. See section “CONFIGURATION” for setup modes.
Note
: communication via RS485 port is disabled if the instrument is connected to the
PC through the mini-USB output.
The
DeltaLog14
software package includes utility files to make RS485 communication
easier, see guide “Utility program for RS485 connection” included in the package. The
following section describes the complete protocol.
8.1 MODBUS-RTU protocol
This protocol is “
Master-Slave
” type. The network includes only one “Master” device,
typically a PC, the other units, the transmitters, are all “Slave” type. The “Master” unit
can transmit commands and requests of data to the “Slave” devices on the network. A
“Slave” device communicates only with the “Master” unit following a request from the
latter. Direct communication between “Slave” devices is not allowed; additionally,
“Slave” devices cannot transmit data on the line if not requested.
Commands transmitted by a PC to the instrument consist of four fields:
Fig. 8.B: Data packet format in MODBUS-RTU protocol
Address
(1 byte)
Function
(1 byte)
Data
(0 to 252 bytes)
CRC
(2 bytes)
