WATLOWMLS300 Series User’s Guide

common to chassis ground. When such a PC is connected to 

the controller, this can provide a path to ground for current 

from the process that can enter the controller through a sensor 

(such as a thermocouple). This creates a ground loop that can 

affect communications and other controller functions. To 

eliminate a ground loop, either use an optically isolated 

communications adapter or take measures to ensure that 

sensors and all other connections to the controller are isolated 

and not conducting current into the unit.

Power Connections

This section covers making the power connections between 

the MLS300 components and testing those connections before 

completing sensor and controller wiring in the following 

sections.

Figure 2.15 on page 36 and Figure 2.16 on page 36 illustrates 

the power connections.

Figure 2.15 Power Connections with MLS300-

AIM

Figure 2.16 Power Connections with CIM300

At this point you have isolated the parts of

the MLS300 that can be damaged by excess

voltage. 

4. With ac power on, use a voltmeter to measure the

following:

(a) The voltage between the COM and +V terminals

on TB 1 should be +12 to 24 Vdc

(b) The voltage between COM and EX should be 0

Vdc

5. If the voltages are not as described in Step 4, check the

installation of the power supply, troubleshoot or replace

the power supply. If the voltages are within the limits

described in Step 4, continue to Step 6.

6. Turn off the power and plug TB1 back into the MLS300-

PM. 

7. Turn the power back on. The Processor Module's display

should light up, and after about a second the Bar Graph

display should appear, followed by the message AIM

COMM FAIL.

8. Verify power to the MLS300-AIM. With a voltmeter,

measure the following:

(a) The voltage between +5 IN and PWR COM terminals on TB-3 on the MLS300-AIM should be

+4.75 to +5.25 Vdc.

9. If the voltage is not as described in Step 8, check the

wiring from the MLS300-PM to the MLS300-AIM. If

the voltage is within the limit described in Step 8,

continue to Step 10.

10. Turn off the power and carefully insert the AIM cards

back into the AIM Terminal Block.

11. Reconnect the AIM communications cable.