Rice Lake EL232 XPCD Explosion Proof Remote Serial Display - Manual (Page 20)

16 EL232 XCPD Installation Manual
3.2.3 20 mA Passive Current Loop Input
In this configuration, an external voltage or current source is connected to a current switch in the sending
instrument. Data is transmitted as a current and is passed through an optical isolator. This current causes an LED
to emit light onto a photo transistor, which converts this light into a TTL signal. Current transmission can be
made over long distances and are quite immune from transient voltage 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. Normally a current loop should not send data at speeds great than 4800 baud.
A passive current loop means that the instrument which sends the data does not supply the voltage to power the
loop. The voltage to power the loop must come from an external power supply or receiver. This voltage can be a
small as 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 only drive two drops. If the same loop were powered by 24
VDC, it could power about 11 remote displays.
The EL232 XPCD can supply power to a passive current loop. This voltage supplies about 8 VDC for use in the
loop.
Determine that your indicator has a passive current loop and connect it as shown in Figure 3-7. Strip the wires
approximately 3/16" and insert them into the connector as shown. Tighten the connectors and test the tightness
by gently pulling on the wires.
Figure 3-7. 20 mA Passive Current Loop Hookup
Note: The serial output is not available when using this input type without board modifications. Consult the factory for
details.
GND PRINT K4 K3 K2 K1 +5V CL4 IN- CL4 IN+ CL3 IN- CL3 IN+ CL2 IN- CL2 IN+
CLO OUT
-
CLO OUT(+)
J2
CL (-)
CL1 (+)
RS-232 (+)
RS-232 (-)
Install this jumper