Dear Experts,

Appreciate if you can give your comments on paralleling  of two transistor digital outputs ( say I want to parallel Q0.0 of CPU1214  (PLC#A) with Q0.0 of CPU1214  (PLC#B)  to drive a solenoid valve rated  5W @24VDC. Either of output  should drive the solenoid or both can drive i.e a sort of ORing takes place. (The Solenod has built-in freewheeling diode and polarity protection built-in).

My question is :  Can I safely parallel the Q outputs without damaging the Digital Output channels?,  I plan to do the paralleing of Q0.0 ...Q0.7 of PLC#A with respective bits Q0.0 ..Q0.7 of PLC#B . 

Thanks.

Kamath RL

Hello,

I think yes you can connect parrarel DO if there are protections like you said diodes.. Just keep mind what for both controller's and other system which will be controlled must be connected to same M (-24V)

Hello.

I would be careful having two PLCs power the same coil. It can be done if both PLCs have the same DC power source. Having both PLCs on the same (-) reference is not good enough in my opinion.. especially when the distance is not short.

Then there is also the question on where the coil is referenced.

On the safe side, consider using 1 or 2 relays. Use 2 relays if the (-) reference of the coil is not the same as the PLCs. Use 1 relay if one PLC has the same reference.

A cheaper way to protect the PLC is to use a diode. But I have seen that in the field the guys do not expect it and causes problems for them later on. A relay is something they understand.

~

Thanks to all those who took interest in answering my question. I would like to add some comments further.

1. Limited space to add the relays for every pair of paralleling Q0.x due to limited space available  in the Ex-d enclosure  chosen in the design. I have six pairs Q0.x  to parallel. 

2. Paralleling is to achieve parallel redundancy of outputs for my application.

3. Siemens support has also answered it. Please refer to the attachment sent by Siemens Support..

Again thanks.

Regards

Kamath RL

Very interesting project to use 2 basic PLCs to achieve redundancy but without synchronization.

However, if you want or need synchronization, that same Siemens doc sect 11.8 shows it can be achieved with lots of cross PLC A dig-out -> PLC B dig-in, and PLC B dig-out -> PLC A dig-in.  Then outputs could be relatively closely synchronized.

Do you have a 3rd PLC monitoring the 2 redundant PLCs for proper operation?  Just curious how far you have gone down the path.

==========

Thanks Einstien,

I plan to use Synchronization between two CPUs by using :

1. One of the Q0.x designated as heartbeat in each CPU which fed to both CPUS as input I0.0 and I0.1 to derive alogic state on the health of each CPU.

2.The two CPU are interconnected via ethernet cable through PN port for exchange  , a synchronization DB for exchange and verification of I/O image and Setting parameters and current data at the end of each scan. 

3. Further logic under development. But I do not see any problem to achieve a parallel redundancy in case of two healthy CPUs or Takeover in Mono mode in case one CPU drops out. Third PLC may not be necessary. (Also the space of 530mm x 330 mm of mounting plate is limited in Ex-d enclosure considering I have to house a incoming Surge arrester , 3 off 2-pole AC incoming , 2 x 24 VDC Power Supply , A redundancy Coupler , Cartridge fuses, A Ethernet hub , 6 relays). 

Thanks for your interest.

Kamath