RG-TDMOD 4
Introduction
2.2.3 Network Termination
For an RS-485 network it is essential to install the specified termination resistors (120
W
), i.e. one at both
ends of a network segment. Failure to do so will result in signals being reflected back along the cable which
will cause data corruption. An external 120
W
1/4W resistor can be connected as shown in Figure 3.
AIN
AIN
COM
U
V W
L1 L2
9 A+ A1
20
20
E1 E2 E3
71
62
7
72
COM
COM
+12
V
+15
V
+15
V
+10
V
8U
28 7
L3
E4 A2
TXA
TXB
COM
8I
Connect to
drive earth
(PE)
120
Ω
1/4W
Figure 3: Network Termination Resistor
RS-485 half-duplex interface allows up to 32 devices to communicate on the network using a twisted
pair of wires. The wires must be terminated at both ends of the network with resistors equal to their
characteristic impedance, typically 120
W
. In noisy environments, twisted and shielded wire should be
used. Ground the shield at the drive end only. This will further reduce unwanted noise and improve overall
communication reliability. In addition, grounding terminal 7 on the
Tmd
is recommended when using serial
communications.
2.3 Modbus Details
A.
Tmd
Drives running the Modbus communication protocol use the RTU (Remote Terminal Unit)
transmission mode and are slaves only. Therefore, the device communicating with the drives must be
a Modbus Master. The baud rate is 9600. By default the Tmd uses no parity and two stop bits. There
are provisions for Odd parity, 1 stop bit and Even parity 1 stop bit as well. The bit sequence is:
DATA
Start bit
1
2
3
4
5
6
7
8
Stop bit
Stop bit
B. At this time the
Tmd
drive does not support the broadcast function of the protocol.
C. IMPORTANT NOTE: Modbus 3X and 4X Registers are numbered starting at 1. However, when transmitted
to a slave over the serial link, the actual address transmitted is one less. This is because the addresses
are numbered starting from 0. The
Tmd
register numbers are also numbered starting from 0. Therefore,
Tmd
register numbers always correspond exactly with the address transmitted. As a result, MODBUS
REGISTER NUMBERS ARE ALWAYS ONE GREATER THAN
Tmd
REGISTER NUMBERS. WHENEVER THE
WORDS “REGISTER #xx” APPEAR, IT SHOULD BE ASSUMED THAT THEY MEAN “
Tmd
REGISTER xx”
and the Modbus Register number will be one larger. In some instances we may show both for clarity.
For example: “Register #24 (Modbus Register #25) . . .”
