MJ-4 Communications Module
7
In an RS-485 application, connect Signal Return (RTN) between the communication devices when there is a ground
potential rise between the connected devices. When the device RTN pins are connected, jumper J11 should be Out
(unless required by safety codes) to prevent ground loop currents. (If it is necessary to connect the RTN to GND
after the Communications Module is installed, you can make this connection at the 8-pin terminal block connector
with a wire jumper.)
Table 4-2 RS-232/485 Interface Jumper Settings
Jumper
Name
Position Function Selected
State Description
1-2
Receive Enable
RS-485 receiver is always enabled
J10 RS-485
RCV EN
2-3
Auto Enable
RS-485 receiver is automatically enabled by MJ-X
Communications Module microprocessor control.
Out
Not grounded
Signal Return not connected to frame ground
J11 Safety
Ground Strap In
Grounded
Signal Return connected to frame ground
Note:
Default pin settings are shown in bold text.
The Jumpers for the Fiber Optic and RS-232/485 interfaces are specific to their operation or functionality, i.e. J14
and J12 (Fiber Optic interface Jumpers) will not affect the working of the RS-232/485 interface and J10 and J11
(RS-232/485 Jumpers will not affect the working of the Fiber Optic interface. Though the jumpers J14 & J12 and
J10 & J11 may be placed in the required operational position, only one interface can be used at a time.
4.4 Replacing the MJ-4 Rear Cover
Remove the adhesive strip covering the rectangular opening for the RS-232/485 Connector, and remove the two
plastic caps covering the holes for the Fiber Optic "In" and "Out" connectors. Make sure that all of the cables and
wires are inside the enclosure and carefully place the rear cover over the communications module and main PCB
and fasten with the four case screws. Uncurl the cable clamp and slide the cable into place.
4.5 Mounting the Communications Module Label
Remove the backing from the label and place it on the rear cover. Make sure that the holes in the label are lined up
with the indicator and connector holes on the rear cover. When the label is in place, rub the label to secure it in
position. The rear cover with labels installed is shown in Figures 4-5 below.
Rxd
Txd
POWER
Watchdog
Communications
RS - 232 / 485
Communications Module
G
N
D
R
T
N
-
+
R
x
d
T
x
d
C
T
S
R
T
S
Data
IN
OUT
l
l
Figure 4-5 MJ-4 with MJ-4 Communications Module Label Installed.
Summary of Contents for MJ-4A
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