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, instal l a j umper on W6, see Tab le 2–12.
Table 2–12. RS-485 Termination using W6

W4 Result

On Connects a 100 ohm termination resistor acr oss 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-48 5 mult idrop ins tallatio ns,
standard RJ11 modular telephone connector jacks are often used to attach standard twisted-pair
cables between systems.
RJ11 modular connectors have 6 available contact po sition s, but only 4 are po pulate d. The 4 cen ter
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 T able 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. Inst allin g a j umper o n W6 con nects a termin ation
resistor across the RS-485 line on the Little Board/486e CPU.