TPU2000/2000R Modbus/Modbus Plus Automation Guide
11
Section 3 - TPU2000 and TPU2000R Device Connectivity
Communication between devices is only possible through connectivity of the units through a physical media
interface. There are two physical interface types on a TPU2000R and a TPU2000. Table 3-1 lists the
characteristics for each of the port types. Those physical interfaces are:
RS232 (isolated and non-isolated)
RS485 (isolated)
Table 3-1. Physical Interface Options
TPU2000R TPU2000
Notes
COM 0
RS232 Non Isolated
RS232 Isolated
Front Port Standard 10 Byte
COM 1
RS232 Non Isolated
Standard 10 Byte Only
COM 2
RS232 Non Isolated
Standard 10 Byte Only
COM 3
RS232 Isolated/RS485
Isolated or Modbus Plus
RS232 Isolated
( NOT
OFFERED)
TPU2000R – Communication Option Card
Determines Physical Interface
AUX
COM
10 base FL port and
ethernet copper
connection OR RS485
(Isolated) and/or INCOM
(2 Winding Unit Only)
RS485 (Isolated)
and/or INCOM
RS485 (Isolated) and/or INCOM
RS232 Interface Connectivity
RS232 is perhaps the most utilized and least understood communication interface in use. RS232 is sometimes
misinterpreted to be a protocol; it is in fact a physical interface. A physical interface is the hardware and network
physical media used to propagate a signal between devices. Examples of physical interfaces are RS232 serial
link, printer parallel port, current loop, V. 24, IEEE Bus… Examples of network media are, twisted copper pair,
coaxial cable, free air…
RS232 gained widespread acceptance due to its ability to connect to another RS232 device or modem. A modem
is a device, which takes a communication signal and modulates it into another form. Common forms of modems
include telephone, fiber optic, microwave, and radio frequency. Modem connectivity allows attachment of
multiple devices on a communication network or allows extension of communication distances in a network with
two nodes. Physical connection of two devices or more than two devices require differing approaches. Figure 3-1
illustrates a topology using two devices (point to point topology). Figure 3-2 illustrates a multi-drop topology
between many nodes. RS232 was designed to allow two devices to communicate without using intermediate
devices.
Port Isolation
Network installation within a substation requires special considerations. A substation environment is harsh in that
high levels of electromagnetic interference are present. Additional ground currents are present in such
installations. RS232 is an unbalanced network in that all signals are referenced to a common ground. On
longer cable runs, the potential of the signals at the sending device can be significantly lower than at the receiving
end due to electrical interference and induced ground current. This increases with long runs of cable and use of
unshielded cable. ABB’s Substation Automation and Protection Division recommends the length of RS232 cable
be less than 10 feet (3 meters) for an un-isolated port and that the cable be shielded. Internal to a typical device,
the RS232 transceivers are referenced to the electronic components internal ground. Any electrical interference
could be coupled through the chip set and fed back to the device. Typical isolation ratings of a non-isolated port
could be as low as 1 volt. Such a port could allow electrical feedback of noise to the electronics for any signal
interference over 1 volt.
Coms 0 through 2 on TPU/TPU/GPU units are non–isolated. However an RS232 implementation on Com 3 uses
opto-isolation technology which increases electrical isolation from the port to the devices internal circuitry to 2.3
kV. It is highly desirable to utilize this port in connection to devices in longer cable runs and dedicated
communication networks. RS232 isolated ports are limited in connection distance for a maximum of fifty feet.