Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 312
Analog-Input Expansion Modules, 2.2 Pulse-Input Expansion Modules, 2.3 Serial-Input Expansion
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Section 8. Operation 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. Onboard 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: Vo = 4.9 V ‐ (330 Ohms) * Io Where Vo is the drive limit, and Io is the current required by the external device. Figure Control Port Current Sourcing (p. 313) plots the relationship. 312
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