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User’s Manual
38 ECB-865 User’s Manual
3.11.20 Signal Description – Serial Port 1 / Serial Port 2 in RS-485 Mode (J9, J12)
RxD/TxD +/- Bi-directional data signal pair.
Received data is available in Serial Port 2 Receiver Buffer Register.
Data is transferred from Serial Port 2 Transmit Buffer Register to the communication line, if
the TxD line driver is enabled through the Serial Port 2’s DTR signal (Modem control
register). The data transmitted will simultaneously be received the in Serial Port 2 Receiver
Buffer Register.
CTS/RTS +/- Bi-directional control signal pair.
The level of this differential signal pair could be read from the Serial Port 1’s CTS signal
(Modem control register). The level of this differential signal pair could be controlled through
the Serial Port 2’s RTS signal (Modem control register).
Warning: Do not select a mode different from the one used by the connected peripheral,
as this may damage CPU board and/or peripheral.
The transmitter drivers in the port are short circuit protected by a thermal
protection circuit. The circuit disables the drivers when the die temperature
reach 150 °C.
RS-422 mode is typically used in point to point communication. Data and
control signal pairs should be terminated in the receiver end with a resistor
matching the cable impedance (typ. 100-120 Ω). The resistors could be placed
in the connector housing.
RS-485 mode is typically used in multi drop applications, where more than 2
units are communicating. The data and control signal pairs should be
terminated in each end of the communication line with a resistor matching the
cable impedance (typical 100-120 Ω). Stubs to substations should be avoided.