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Little Board™/486e Technical Manual
2–16
The RS-485 interface specification requires that both ends of the twisted-pair cable be terminated
with 100-ohm resistors. You can terminate the RS-485 interface on J6 with a resistor provided on
the Little Board/486e CPU. To terminate the line, install a jumper on W6, see Table 2–12.
Table 2–12. RS-485 Termination using W6
W4 Result
On Connects a 100 ohm termination resistor across J10
Off No termination
RS-485 Twisted-Pair Cabling Using RJ11 Connectors
Connector J6 is used for an RS-485 twisted-pair connection. In RS-485 multidrop installations,
standard RJ11 modular telephone connector jacks are often used to attach standard twisted-pair
cables between systems.
RJ11 modular connectors have 6 available contact positions, but only 4 are populated. The 4 center
conductors are wired so that the two outside and the two inside conductors are connected together.
This eliminates any confusion about pin numbering conventions, as a reversal of connections has no
effect. In addition, the lines were chosen to minimize the possibility of circuit damage should the
unit be accidentally plugged into a standard telephone outlet. It sets the phone line to its offhook
state to prevent the phone from ringing.
The recommended wiring for a J6-to-RJ11 cable is shown in Table 2–13.
Table 2–13. J6/RJ11 Cable Wiring
J6
Pin
RJ11
Pin
Signal Standard
Wire Color
1N/C
2 2 - I/O Signal Black
1 3 + I/O Signal Red
1 4 + I/O Signal Green
2 5 - I/O Signal Yellow
6N/C
When connecting the RS-485 port into a multidrop network, the devices at both ends of the network
should be terminated with a 100 ohm resistor. Installing a jumper on W6 connects a termination
resistor across the RS-485 line on the Little Board/486e CPU.