Sensoray 7429 - Manual (Page 32)

CONNECTIONS
G ENERAL
All sensors are connected to the 7429 through the 40-pin ribbon cable connectors P1
and P2. Optionally, sensors may be connected to the model 7409TB screw termination
boards which in turn connect to the P1 and P2 connectors via 40-conductor ribbon ca-
bles. Sensors may be electrically connected and disconnected from the 7429 or 7409TB
boards with power applied to the STDbus.
Each sensor has as many as five connections to a channel circuit. Two connections are
universally used for all sensor types: SENSE+ and SENSE-. These two pins are the
positive and negative differential voltage sense inputs, respectively. In addition to the
sense pins, all resistive sensors (i.e., RTD's, thermistors, and strain/pressure gauges) re-
quire connection to the POWER+ and POWER- pins.
The POWER+ and POWER- pins provide positive and negative excitation, respective-
ly, for all resistive sensors. This excitation is supplied in the form of a strobed DC volt-
age (~10V) for strain and pressure gages, and pulsed constant current for all other
resistive sensors. The fifth lead is intended for connection to a cable shield.
The remainder of this chapter describes how to connect sensors to the 7429 for proper
operation.
RTD S
RTD's can be connected to the 7429 in three possible configurations: two-wire, three-
wire, and four-wire.
The simplest configuration is the two-wire circuit. As the name implies, this configu-
ration requires only two wires. The SENSE+ and POWER+ pins are shorted together,
and SENSE- and POWER- are shorted together. The drawback to this circuit is that a
significant voltage may develop across the sense wires if you run a long cable out to the
RTD. Depending on the length of cable, this voltage will introduce error into the RTD
measurement. There are two solutions to this problem: use shorter cable or use more
than two wires.
Two-wire connection
By using more than two wires it is possible to run one or both of the sense wires all the
way out to the RTD. This enables the 7429 to sense RTD voltage at the RTD instead
of at the 7429 connector. Since the SENSE pins are high impedance inputs, virtually
Sense+
Sense-
Power+
Power-
Shield