Sensoray 118 - Manual (Page 6)

Sensoray Model 118 Smart A/D™ Instruction Manual 4 Hardware Configuration
3 Hardware Configuration
The Smart A/D™ board requires the configuration of
switches and programming shunts to select hardware
options. These options must be configured before
installing the Smart A/D™ board into the target system.
Programming shunts are configured by installing or
removing shorting shunt plugs at various locations on the
Smart A/D™ circuit board. A supply of programming
shunts, of sufficient number to program all board options,
is included with the Smart A/D™ board.
3.1 Base Address
The Smart A/D™ board may be mapped to any 256-byte
block within the standard VMEbus short supervisory and
nonpriveleged address spaces (Address Modifier Codes
2D and 29). Custom address modifier Codes can be
supplied by special order; contact Sensoray for details.
The board occupies a 256-byte block of space in both
address spaces, but it uses only the first two addresses in
the block. To avoid address conflicts, you must not map
any other devices into the address spaces occupied by the
Smart A/D™.
Rotary switches S1 and S2, located on the board as
shown in Figure 2, select the base address for the board.
These switches are factory set to locate the board at
address B200H (hexadecimal). You must change the
positions of switches S1 and S2 if you require a different
base address.
Switch positions may be changed by inserting a small
screwdriver into the slot on the face of the switch and
then gently rotating the slot.
Figure 3: Base Address selection switches
As shown in Figure 3, switch S2 (labeled “HIGH” on the
circuit board silkscreen) sets the most significant hex
address digit, and S2 (labeled “LOW”) sets the least
significant digit.
For example, to select address 3700H, set switch S2 to
the “3” position and S1 to the “7” position.
3.2 Signal Conditioning Circuit (SCC)
Each channel is provided with a signal conditioning
circuit (SCC) which may be inserted into the sense signal
path. The SCC performs these functions:
q Common-mode tie-down. This function helps to prevent
the common mode voltage (CMV) from exceeding the
input CMV range of the Smart A/D™ measurement
section. This function is required when a sensor channel is
driven from an isolated source, such as a battery, isolated
power supply, or thermocouple.
q
Open-sensor detect. If either of the two sense signals
should become disconnected from the source, this function
forces a differential voltage of 700mV, minimum, to
appear across the sense inputs. This function is useful
when a sensor is used in a control loop.
q
RF shunt. This function, which shunts RF noise to
ground, is essential when connecting to isolated sources,
such as thermocouples.
3.2.1 Side Effects
Sense input impedance is reduced when a channel’s SCC
is enabled. See “General Specifications” on page 35 for
details.
3.2.2 Recommended Settings
The decision of whether or not to enable a channel SCC
is influenced primarily by the type of sensor to be
interfaced, and, in the case of active sources, whether or
Table 1: default configuration from factory
Option Default
Base address B200 (hex)
Channel SCC’s All disabled
MSB LSB