Ideas in Measuring ..
TECHNIQUES LIMITED
Page 6
LDU68 Series Manual
Standard Unit with RS422 Point to Point Connections
Standard Unit with RS485 Multi-drop Connections
Top view
Exc
6
7
Gnd
Gnd
Gnd
9
Pwr
Out
In
Out
In
Tx
Tx
Rx
Rx
+
+
0
1
-
-
0
1
8
Sen
5
Inp
4
Inp
3
Sen
2
Exc
1
0
16
17
19
18
15
14
13
12
11
10
+
+
+
-
-
-
I I I
Load Cell Digitising Unit
LDU 68.1
Cert.no. DK0199 - R76 - 10.04
n
max
= 10000
-15°C/+55°C
CE
D
min = 1.0
m
V/VSI
NC
NC
TECHNIQUES LIMITED
www.sensortechniques.com
Tx+
Tx-
Rx+
Rx-
Signal gnd
+12 to 24V
Gnd
Power Supply Gnd
+ Power Supply
Signal Gnd
Power Supply Gnd
Rx (Pin 10) to +Tx (Pin 13)
and -Rx (Pin 11) to -Tx (Pin12)
Exc
6
7
Gnd
Gnd
Gnd
9
Pwr
Out
In
Out
In
Tx
Tx
Rx
Rx
+
+
0
1
-
-
0
1
8
Sen
5
Inp
4
Inp
3
Sen
2
Exc
1
0
16
17
19
18
15
14
13
12
11
10
+
+
+
-
-
-
I I I
Load Cell Digitising Unit
LDU 68.1
Cert.no. DK0199 - R76 - 10.04
n
max
= 10000
-15°C/+55°C
CE
D
min = 1.0
m
V/VSI
NC
NC
TECHNIQUES LIMITED
www.sensortechniques.com
Signal Gnd
Power Supply Gnd
Rx (Pin 10) to +Tx (Pin 13)
and -Rx (Pin 11) to -Tx (Pin12)
Fit a 120 Ohm terminator to the last
device on the bus
RS485 Interface
120 Ohm
120 Ohm
R1
R2
In multi-drop configuration, there
can be brief periods when no
transmitter is enabled, and the
network is therefore allowed to
float. To avoid this, pull up/pull
down resistors (R1 & R2) can be
added. Typical values of R1 & R2
for a 12V supply voltage will be
1K2 Ohms. For lower supply
voltages such as 5V, the values of
R1 & R2 can be lowered to 470
Ohms.
Fit 120 Ohm terminator
at host end
3 HARDWARE & WIRING DIAGRAMS
Issue 1h
Bottom view
Sw1 & Sw2:
Close for 4-wire
load cell
Sw3: Close to enter
configuration mode
Sw5: Close to connect
power supply ground to
chassis
Sw4: Do NOT close.
Used for program
download only.
Sw3
Sw5
Sw4
Ideas in Measuring ..
TECHNIQUES LIMITED
Page 7
LDU 68 Series Manual
The load cell(s) are connected to pins 0 to 6 of the LDU 68.x.
Pins 1 & 6 provide excitation (5 V DC) to the load cell circuit, pin 1 being positive excitation (+Exc)
and pin 6 negative excitation (-Exc).
Pins 3 & 4 are the signal inputs to the LDU 68.x from the load cell circuit, pin 3 being the positive
signal (+Sig) and pin 4 being the negative signal (-Sig).
Pins 2 & 5 are the Sense connections for the LDU 68.x, pin 2 being the positive sense (+Sen) and
pin 5 the negative sense (-Sen). If your load cell has 6 wires ( 2 for Excitation, 2 for Signal and 2 for
Sense) the sense connections provide the LDU 68.x with the voltage that reached the load cell(s)
as opposed to the voltage that was sent. This enables the LDU 68.x to correct for voltage drops on
long cables.
If your load cell only has 4 wires you will need to link the positive excitation (+Exc) pin (Pin 1) and
the positive sense (+Sen) pin (Pin 2) and similarly the negative excitation (-Exc) pin (Pin 6) and the
negative sense (-Sen) pin (Pin 5). This can be achieved simply by solder bridging pads SW1 and
SW2 on the underside of the LDU 68.x pcb. (See diagram on Page 6)
Pin 0 provides a ground connection for the load cell cable screen or drain wire.
Power is provided to the LDU 68.x via pins 18 & 19, pin 18 being the positive supply (Pwr) 12 - 24
VDC and pin 19 being the power supply ground (Gnd) 0 V.
3.1 Load Cell Connections
3.2 Power Connections
3.3 Logic Inputs and Logic Outputs
3.4 Communication Connections
3.4.1 RS422 Point to Point 4 wire connection half or full duplex.
The standard interface on the LDU 68.x is RS422. The Tx+ connection from the host system is
connected to +Rx (Pin 10) of the LDU 68.x. Similarly the Tx- from the host is connected to -Rx (Pin
11) , the Rx- is connected to the -Tx (Pin 12) and the Rx+ is connected to the +Tx (Pin 13). The
shield connection for the RS422 cable should be connected to the power supply ground Gnd (Pin
19)
3.4.2 RS485 Multi-Drop 2 wire connection half duplex.
The LDU series can be wired in a multi-drop mode where up to 32 devices can be connected to one
bus. The +Rx (Pin 10) and +Tx (pin 13) should be commoned together and should be connected to
the A (Tx-/Rx- or D-) terminal of the host RS485 interface. The -Rx (Pin 11) and the -Tx (Pin 12)
should similarly be commoned and connected to the B (Tx+/Rx+ or D+) terminal of the host RS485
interface. The ground connection (GND) from the host RS485 should be connected to the power
supply ground of the LDUs. Termination resistors of 120 Ohm each should be placed between the B
(Tx+/Rx+ or D+) and A (Tx-/Rx- or D-) lines at host end of the bus and between the B (Tx+/Rx+ or
D+) and the A (Tx-/Rx- or D-) lines of the last device on the bus. In addition it may be necessary to
add pull up and pull down resistors (R1 & R2) to the B (Tx+/Rx+ or D+) and A (Tx-/Rx- or D-) lines to
prevent these lines ‘floating’ during periods of no transmission. Where the supply voltage to the
RS485 line driver IC is typically 5V DC for example, the value of R1 & R2 can be 470 Ohms each.
Where for example you are using a RS232 to RS485 converter with a supply voltage of 12 V DC,
the value of R1 & R2 should be increased to 1K2 Ohms (1200 Ohms).
The LDU 68.x have 2 logic inputs on pins 14 & 16 and 2 logic outputs (Open Collector) on pins 15 &
17. The common for both the logic inputs and outputs is internally connected to the ground of the
power supply so no separate common supply is required. Control signals (10 - 30 V DC) applied to
the logic inputs can be used to trigger a variety of weighing processes such as check-weighing,
filling etc. The status of the logic inputs can be read via the LDU 68.x communication port. The logic
outputs are effectively weight dependant switches. These can be used to control alarms or filling
valves etc. The load to be controlled is wired between the positive supply and the appropriate logic
output terminal on the LDU 68.x.
Issue 1h
B or D+ or Tx+/Rx+
Gnd
A or D- or Tx-/Rx-
V
cc
Exc
6
7
Gnd
Gnd
Gnd
9
Pwr
Out
In
Out
In
Tx
Tx
Rx
Rx
+
+
0
1
-
-
0
1
8
Sen
5
Inp
4
Inp
3
Sen
2
Exc
1
0
16
17
19
18
15
14
13
12
11
10
+
+
+
-
-
-
I I I
Load Cell Digitising Unit
LDU 68.1
Cert.no. DK0199 - R76 - 10.04
n
max
= 10000
-15°C/+55°C
CE
D
min = 1.0
m
V/VSI
NC
NC
TECHNIQUES LIMITED
www.sensortechniques.com
Inp
3
2
Gnd
NC
Sen
0
+Pwr
Out
Rx
+
Tx
+
In
0
In
1
Rx
-
Tx
-
0
Out
1
1
Inp
4
Sen
5
Exc
6
Gnd
7
NC
8
9
12
13
11
10
15
14
17
16
19
18
+
+
-
-
-
-
I I I
Load Cell Digitising Unit
LDU 68.1
Cert.no. DK0199 - R76 - 10.04
n
max
= 10000
-15°C/+55°C
CE
D
min = 1.0
m
V/VSI
Exc
Gnd
TECHNIQUES LIMITED
www.sensortechniques.com
Sw2
Sw1
