Ampro Corporation 486E manual RS-485 Twisted-Pair Cabling Using RJ11 Connectors

Little Board™/486e Technical Manual

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

	1	N/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
	6	N/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.

2–16