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TS-5700 User's Manual
5.3 RS-485 Support
RS-485 Quick Start Procedure:
1. The RS-485 option must be installed
2. Install FD jumper for full-duplex or HD for half-duplex RS-485 operation
3. Attach the RS-485 cable to the 10-pin Header-Adaptor cables to Sub-DB9 connector are available from
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4. Set the COM1 UART serial parameters (baud rate, data, parity, and stop bits, interrupts, etc).
5. Run Auto485.exe utility(configures bits 6 and 7 at I/O 75h)(and initializes Timer2)
6. For Half-Duplex RTS mode: To transmit data, assert RTS and write the data to the UART. To receive
data, deassert RTS and read the data from the UART
7. For Half-Duplex Automatic mode: just read or write data to the UART
An option is available to add support to COM1 for half duplex or full duplex RS-485. RS-485 drivers allow
communications between multiple nodes up to 4000 feet (1200 meters) via twisted pair cable. Half-duplex RS-
485 requires one twisted pair plus a Ground connection, while full duplex requires two twisted pair plus a
Ground.
For half-duplex operation, a single twisted pair is used for transmitting and receiving.
Bit 6 at I/O location 75h must be set to enable RTS mode or bit 7 can be set to enable
Automatic mode. In RTS mode, the serial port RTS signal controls the RS-485
transmitter/receiver
(See Automatic RS-485 TX Enable
true, the RS-485 transmitter is enabled and the receiver disabled. When RTS is de-
asserted the transmitter is tri-stated (disabled) and the receiver is enabled. Since the
transmitter and receiver are never both enabled, the serial port UART does not receive
the data transmitted.
For full-duplex operation, two twisted pairs are used and the transmitter can typically be left on all the time.
Simply use RTS mode, and set RTS true.
See
Table 5.2b above
Note: the correct jumper (FD or HD) must be installed.
Fail-safe RS-4 bias resistors are used to bias the TX+, TX- and RX+, RX- lines to the correct state when these
lines are not being actively driven. This is an important consideration, since in a typical RS-485 installation, the
drivers are frequently tri-stated. If fail-safe bias resistors are not present, the 485 bus may be floating and very
small amounts of noise can cause spurious characters at the receivers. 4.7KW resistors are used to pull the TX+
and RX+ signals to 5V and also to bias the TX- and RX- signals to ground. Termination resistors may be
required for reliable operation when running long distances at high baud rates. Termination resistors should only
be installed at each end of an RS-485 transmission line. In a multi-drop application where there are several
drivers and/or receivers attached, only the devices at each end of the transmission line pair should have
termination resistors.
A read at I/O location 75h bit 1 will return a "1" when the RS-485 option is installed.
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for connector pin-outs.
below). When RTS is asserted
See Table 5.3
for details.
9
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Table 5.3 COM1Receiver
Source
Jumper
Receiver Source
FD
Full-Duplex RS-422
HD
Half-Duplex RS-485
232
RS-232
5/2009
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