Campbell Scientific CR800 and CR850 Measurement and Control Systems - Manual (Page 312)

Section 8. Operation
312
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 CR800. 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 – C4) can be configured as an output port and
set low (0 Vdc) or high (5 Vdc) using the PortSet() or WriteIO() instructions.
Port C4 can be configured for pulse width modulation with a maximum period of
36.4 s. 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. 313)
plots the relationship.