LB-762 Controller Installation Manual
LAB-EL
Rysunek 3.14: Scheme of correct and incorrect RS-485 network topology
3.11.1
Termination of RS-485 network
RS-485 bus requires relevant termination (impedance match and line polarization). Termination of network should be con-
nected only in two places - on the ends of the network. This means these devices (network nodes) to which cable comes
and does not go further to next devices. On Figure
termination should be enabled in devices described as
LB-762 #1
(extreme device on the left side) and
LB-480
(extreme device on the right side). In other devices, located in the middle part
of the network, termination of the network must be off.
In LB-480 regulator 3 switches are used for network termination, described as
RS-485 TERMINATION
):
•
POL+
- polarization (+),
•
POL-
- polarization (-),
•
TERM
- impedance match.
Rysunek 3.15: RS-485 network termination switches
Switches in the lower position are off, while in the top position – on.
In practice switching on termination means switching on of all 3 switches, while turning off termination means turning off
of all 3 switches. The situation is theoretically possible in which switch TERM or pair POL+ and POL-, but in practice it
is not present.
RS-485 interface in converter LB-480 has similar termination mechanisms of RS-485 network. This can be done using 3
contacts which can be found on the interface plate (available after removing the housing), described as
LINE TERMINATION
.
In order to turn on termination of line in converter LB-480 one should set up all 3 contacts:
•
JP1 -POL+
- polarization (+),
•
JP2 -POL-
- polarization (-),
•
JP3 -TERM
- impedance match.
It is recommended that LB-480 converter be the terminal machine in RS-485 network (thereby that line termination be
switched on in it). Of course possible is such network topology in which LB-480 converter is a device placed in the middle
part of the network – in such case termination must be in other devices and this is a completely correct configuration. However,
positioning LB-480 converter on the end of the network and switching on termination in it ensures correct operation of the
network in the case when other equipment (regulators) may be periodically deactivated – in such case the disabled regulator
does not ensure line polarization. Since LB-480 converter is a device which by principle of system operation is always on,
switching on termination and polarization of line in LB-480 is the most effective way of ensuring proper RS-485 network
operation. However, this requires proper network planning so that this device be placed on its end.
25
