WATLOWMLS300 Series User’s Guide

Never wire bundles of low power Watlow

Anafaze circuits next to bundles of high power ac wiring. Instead, physically separate

high power circuits from the controller. If

possible, install high voltage ac power circuits in a separate panel. 

Wiring Recommendations

Keep the following guidelines in mind when selecting wires 

and cables:

• Use stranded wire. (Solid wire can be used for fixed 

service; it makes intermittent connections when you 

move it for maintenance.) 

• Use #20 AWG thermocouple extension wire. Larger or 

smaller sizes may be difficult to install, may break easily, 

or may cause intermittent connections. 

• Use shielded wire. (The electrical shield protects the 

signals and the MLS300 from electrical noise.) Connect 

one end of the input and output wiring shield to earth 

ground.

• Use copper wire for all connections other than 

thermocouple sensor inputs.

See Table 2.1 on page 33 for cable recommendations.

Noise Suppression 

The MLS300’s outputs are typically used to drive solid state 

relays. These relays may in turn operate more inductive types 

of loads such as electromechanical relays, alarm horns and 

motor starters. Such devices may generate electromagnetic 

interference (EMI or noise). If the controller is placed close to 

sources of EMI, it may not function correctly. Below are some 

tips on how to recognize and avoid problems with EMI. 

For the AIM or CIM300 earth ground wire, use a large gauge 

and keep the length as short as possible. Additional shielding 

may be achieved by connecting a chassis ground strap from 

the panel to the case of the processor module.

Symptoms of RFI/EMI

If your controller displays the following symptoms, suspect 

EMI:

• The controller's display blanks out and then re-energizes 

as if power had been turned off for a moment.

• The process variable does not display correctly.

EMI may also damage the digital output circuit—so digital 

outputs will not turn on. If the digital output circuit is 

damaged, return the controller to Watlow Anafaze for repair.

Avoiding Noise Problems

To avoid RFI/EMI noise problems:

• The MLS300 system includes noise suppression 

circuitry. Some of which is only effective when the 

components are properly grounded. Be sure the 

processor module and AIM (or CIM300) are connected 

to earth ground.

• Separate the 120 or 240 Vac power leads from the low 

level input and output leads connected to the MLS300 

series controller. Don't run the digital I/O or control 

output leads in bundles with 120 Vac wires.

• Where possible, use solid-state relays (SSRs) instead of 

electromechanical (EM) relays. If you must use EM 

relays, try to avoid mounting them in the same panel as 

the MLS300 series equipment.

• If you must use EM relays and you must place them in a 

panel with MLS300 series equipment, use a 0.01 

microfarad capacitor rated at 1000 Vac (or higher) in 

series with a 47 Ω, 1/2 watt resistor across the NO 

contacts of the relay load. This is known as a snubber 

network and can reduce the amount of electrical noise. 

• You can use other voltage suppression devices, but they 

are not usually required. For instance, you can place a 

metal oxide varistor (MOV) rated at 130 Vac for 120 Vac 

control circuits across the load, which limits the peak AC 

voltage to about 180 Vac (Watlow Anafaze P/N 0802-

0826-0000). You can also place a transorb (back to back 

zener diodes) across the digital output, which limits the 

digital output voltage.

The above steps will eliminate most EMI/RFI noise problems. 

If you have further problems or questions, please contact 

Application Engineering

Avoiding Ground Loops

Ground loops occur when current passes from the process 

through the controller to ground. This can cause instrument 

errors or malfunctions.

A ground loop may follow one of these paths, among others:

• From one sensor to another.

• From a sensor to the communications port.

• From a sensor to the dc power supply.

The best way to avoid ground loops is to minimize 

unnecessary connections to ground. Do not connect any of the 

following terminals to each other or to earth ground:

• MLS300 PM: TB1, pin 2 (COM)

• MLS300-AIM: TB3, pin 1 to (PWR COM)

• All A COM terminals on the MLS300-AIM or CIM300

• Power Supply: (COM)

• Pin 3 on the RJ connector

Watlow Anafaze strongly recommends that you:

• Isolate outputs through solid state relays, where possible.

• Isolate RTDs or “bridge” type inputs from ground.

• Isolate digital inputs from ground through solid state 

relays. If you can't do that, then make sure the digital 

input is the only place that one of the above pins connects 

to earth ground.

• If you are using EIA/TIA-232 from an un-isolated host, 

don't connect any other power common point to earth 

ground, or use an optical isolator in the communications 

line. 

Personal Computers and Ground Loops

Many PC communications ports connect the communications