Model 485HF9
Page 13
FIGURE 2.7
EXTERNAL TRANSMITTER ENABLE CONNECTIONS
In external control, the transmitter enable line can be controlled from a programmable bit on
the host computer. If the computer is equipped with a parallel printer port, then D7 of this port
may be used to control the transmitter. Connect a single wire from pin 9 of the Centronics
compatible connector to the /EN terminal of the Model 485HF9 Adaptor. The program must
clear D7 to transmit or set D7 to receive over the RS-485 interface.
2.
RTS Transmit Enable
. The transmitter can alternatively be enabled under control of
the RTS (Request To Send) line of the RS-232 host DTE device. See J1 in figure 2.3 for the
appropriate patching.
In normal RS-232 DTE operation, RTS is raised to its ON condition when the device is ready to
commence transmission. In some circumstances, the use of RTS provides a simple means of
enabling the transmitter, however, check overall system timing before using this control
method.
Further information on half duplex and multi-drop operation is given in the applications section
of this manual.
2.5.3
Selection of Local Echo On/Off in Half Duplex Operation
The factory default configuration of link J3 causes the transmitted data can be echoed to the
receiver in half duplex operation. If this echo is not required, the copper link must be severed
and the solder bridge made across the other gap (see paragraph 2.3). This change ensures that
the RS-485 receiver is enabled/disabled in opposite phase to the transmitter.
For full duplex operation, the link J3 must be in its default condition for the receiver to be
continuously enabled. Echo back is not appropriate to full duplex operation.
2.6
Transmission Line Termination
In RS-422 or RS-485 applications, the two wire transmission lines must be properly terminated,
and a single resistor across the receiver input pair normally provides adequate termination.
The Model 485HF9 Adaptor is equipped with a terminator resistor for the receiver input pair,
and this resistor is connected into circuit by the J2 link on the printed circuit board. See figure
2.3. Factory setting is with the receiver input terminator in circuit, but it can be removed from
the circuit by cutting the link as described in paragraph 2.3. Removal of the link is required in
some multi-drop applications.
TXEN
‘0’ Level (or open
circuit) to Enable
‘1’ Level to Disable
Model
485HF9
Adaptor
EN
1
GND 2
TX 3
TX
4
RX
5
RX 6
GND 7
VDC 8
Summary of Contents for 485HF9
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