Rice Lake RD-300 Remote Display - Manual (Page 11)

Installation 9
2.4.2 20 mA Active Current Loop - SW2-8 On
In this mode data is transmitted as a current and is
passed through an optical isolator. This current
causes an LED to emit light onto a phototransistor,
which converts this light into a TTL signal. Current
transmission can be made over long distances and is
quite immune to transient surges or RFI/EMI noise.
The distance a current loop can transmit is limited by
the voltage source available to the current loop and the
speed of transmission which data is sent. The
maximum baud rate is set by the quality of the
indicator transmitter. The RD-300/RD-232 can
receive data at baud rates of up to 38,400.
An active current loop means that the instrument
which sends the data also supplies the voltage to
power the loop. This voltage can be as small as 3.5
VDC to as much as 24 VDC. When the loop goes
through an optocoupler, a “drop” occurs. This drop is
about 2 VDC. A current loop can power as many
drops as there is voltage to power the loop. For
example, if a current loop is powered by 5 VDC, it can
normally drive two “drops”. If the same loop were
powered by 24 VDC, it could power about 11 remote
displays or other devices. Determine that your
indicator has an active current loop and connect it as
s h o w n i n f i g u r e 6 . Str i p t h e i n c o m i n g w i r e s
approximately 3/16". Press on the white lever to open
the connector. Insert the wire and release the lever.
Test the tightness by gently pulling on the wires.
Tighten the strain relief fitting around the serial data
cable with a wrench or pliers.
The 20 mA receiver in the RD-300/RD-232 is an
HP-4200 high speed current loop receiver. The photo
coupler and requires current swings of less than 4 mA
as a mark and greater than 12 mA for a space. The
output of both the RS-232 and the current loop is sent
through an LED on the circuit board. See Figure 2-6.
This LED will be OFF when nothing is connected to
the board, and will flash when serial data is being
received. Please note that when the LED flashes, it
means that something is being received that looks like
a serial signal. It does not guarantee that the signal is
correct.
Figure 2-6. LED Location
2.4.3 20 mA Passive Current Loop - SW2-8 On
A passive current loops means that the instrument
which sends the data does not supply the voltage
source to power the loop. The voltage must come
from the RD-300/RD-232. To use a passive current
loop, wire the unit in the following way.
1. Jumper TB2-3 to TB2-6 (Gnd to CL- IN).
2. Connect the CL+ from the meter to the CL+
Output (TB2-7) and the CL- Output to TB2-5
(CL+ IN).
3. Strip the incoming wires approximately
3/16".
4. Press on the white lever to open the
connector.
5. Insert the wire and release the lever.
6. Test the tightness by gently pulling on the
wires.
7. Tighten the strain relief fitting around the
serial data cable with a wrench or pliers.
Note: Using this configuration changes the current loop
output to a passive transmitter!