Specifications

Figure 4. RS-485 Multidrop Net work.
Figure 5b. 100 Channel Network.
by the host are examined by every module in the chain. If
a module is correctly addressed and receives a valid
command, it transmits a response on the daisy chain
network. The response is rippled through any other
modules in the chain until it reaches the host.
RS-485 is a communications standard developed for
multi-dropped systems that can communicate at high
data rates over long distances, as shown in Figure 4. RS-
485 is similar to RS-422 in that it uses a balanced
differential pair of wires switching from 0 to 5V to commu-
nicate data. RS-485 receivers can handle common mode
voltages from -7 to +12V without loss of data, making
them ideal for transmission over great distances. RS-485
differs from RS-422 by using one balanced pair of wires
for both transmit and receive. Since an RS-485 system
cannot transmit and receive at the same time it is a half-
duplex system. For systems that require more than a few
modules, long wiring distances, or high speed, we
recommend RS-485.
For instance, to command the motor to go at 1500 RPM
requires an output voltage of 4.666V. This data is difficult
to read and interpret. A solution to this problem is to
scale the input data directly in units of RPM.
The - full scale output of 0V is assigned the value 100
RPM with the MN command. The + full scale output of
+10V is assigned the value 3000 RPM with the MX com-
mand. Once the endpoint values are assigned, all other
data values are interpolated linearly. Now to set the mo-
tor to 1500 RPM requires the analog output command:
$1AO+01500.00
The data is much easier to interpret since the scaling is
in RPM. The actual output voltage is +4.666V.
Scaling data in %
A valve actuator accepts a 4-20mA signal: at 4mA the
valve is fully closed and at 20mA the valve is fully open.
We wish to rescale a D4252 0-20mA module to accept
data of 0% closed to 100% open.
The Minimum (MN) command assigns an input data
value of 0mA to the - full scale output of the module.
Using the two scaling points (4mA = 0%) and (20mA =
100%) and a bit of computation, we find that 0mA inter-
polates to a value of -25%. This value is used in the argu-
ment of the MN command:
$1MN-00025.00
The maximum scaling point of 20mA is assigned the in-
put value of 100%:
$1MX+00100.00
The module is now scaled in percentage of valve open-
ing. To set the valve to 50% opening:
$1AO+00050.00
In this case the D4000 module produces an output of
12mA, opening the valve halfway.