LC6 Joystick VREF Connection

Kevin 2 months ago in Levers updated by Gustav Widén (System support) 1 month ago 3

I have an application that uses 2 x LC6 joysticks for driving a safety circuit. Primary signals are fed into MC41FS and secondary comparison channel into XC21. I used XC21 as there are not enough VIN into MC41 - 8 x 2 = 16VIN (we have space restrictions preventing bigger or multiple MC4 module).

Modules share the same power supply and are located next to each other, but power supply to joystick (VREF+ & -ve) comes from MC41 module. Secondary comparison VIN into XC21 has no VREF-ve. To prevent fluctuations on VIN on XC21 do I link VREF- between MC41 and XC21?

If you want to use LC6 dual output in a safety system it is strongly advised to connect both analog signals of the LC6 to an MC4xFS safety controller or XC4x safety I/O module. Having one of the two LC6 signals connected to an XC21 I/O module (non-Safety certified module) the safety level is strongly reduced as you will see performing the mandatory machine safety calculation!, risking not meeting required Safety Levels   

Yes the XC21 is not a safety module but in some scenarios this can still be used in a safety system to satisfy certain integrity levels, see below some information I received from the IQAN team in 2017 regarding using XC21 alongside an MC3 (the safety master at the time):

Image 3808

We may use 2 x MC41FS to overcome the voltage VREF- connection in which case its not an issue, but I'm exploring options given the safety levels appear to be achievable with an XC21. 

To minimize ground level differences, make sure the -BAT connections on XC21 and MC41 are connected.