Campbell Scientific CR1000 Measurement and Control System - Manual (Page 327)

Section 8. Operation
327
over a proprietary, three-wire serial communications link utilizing channels C1,
C2 and C3.
Read More! For complete information on available measurement and control
peripherals, go to the appendix Sensors and Peripherals
,
www.campbellsci.com, or
contact a Campbell Scientific applications engineer.
8.2.1 Analog-Input Expansion Modules
Mechanical relay and solid-state relay multiplexers are available to expand the
number of analog sensor inputs. Multiplexers are designed for single-ended,
differential, bridge-resistance, or thermocouple inputs.
8.2.2 Pulse-Input Expansion Modules
Pulse-input expansion modules are available for switch-closure, state, pulse-count
and frequency measurements, and interval timing.
8.2.3 Serial-Input Expansion Modules
Capturing input from intelligent serial-output devices can be challenging. Several
Campbell Scientific serial I/O modules are designed to facilitate reading and
parsing serial data. Campbell Scientific recommends consulting with an
applications engineer when deciding which serial-input module is suited to a
particular application.
8.2.4 Control Outputs
Controlling power to an external device is a common function of the CR1000.
On-board control terminals and peripheral devices are available for binary (on /
off) or analog (variable) control. A switched, 12-Vdc channel is also available.
See Switched Unregulated (Nominal 12 Volt)
(p. 86).
8.2.4.1 Digital I/O Ports
Each of eight digital I/O ports (C1 – C8) can be configured as an output port and
set low (0 Vdc) or high (5 Vdc) using the PortSet() or WriteIO() instructions.
Ports C4, C5, and C7 can be configured for pulse width modulation with
maximum periods of 36.4 s, 9.1 s, and 2.27 s, respectively. A digital-I/O port is
normally used to operate an external relay-driver circuit because the port itself has
limited drive capacity. Drive capacity is determined by the 5-Vdc supply and a
330-ohm output resistance. It is expressed as:
V
o
= 4.9 V ‐ (330 Ohms) * I
o
Where V
o
is the drive limit, and I
o
is the current required by the external device.
Figure Control Port Current Sourcing
(p. 328)
plots the relationship.