Doug Hylton
Member
Don’t put logic in the I/O wiring
I know that its tempting to save some money and hardwire some contacts, selector switches and other stuff between the I/O device and the PLC but resist this temptation.
If you hardwire logic in your I/O wiring the chances are good that during start up you will be out there on the machine pulling wire instead of changing the program. Run everything directly to the devise or sensor. It makes the wiring simple and you will never regret it.
I first started programming PLC’s on pulp bale finishing lines and all of the manual jog buttons were hard wired to the motor starters and only the automatic mode was done by the PLC. The idea was that if the PLC failed you could run the machine with the pushbuttons. I have seen I/O modules fail but I have never seen a PLC fail except for one and that one was connected to 220 Volts and it did smoke.
The problem with only part of the system wired to the PLC was that during start up we always ran into something we couldn’t do because the device was not wired to the PLC.
Sinking or sourcing
I personally like sinking the inputs and sourcing the outputs. I think that sinking inputs are less susceptible to noise. A sourcing input is sitting there low all the time waiting for a high and it could get the high off of a wire laying next to it from a solenoid valve when the field is collapsing. A sinking input is sitting there high and its waiting for a low. I would source the outputs just to make it less confusing. The Japanese like every thing sinking and you see it more in the electronics field.
What happens when you hit the ESTOP
There are two schools of thought here. One is to just kill everything and the other is to leave the processor and inputs powered up and only kill the output power. Just killing the output power has an advantage if the machine parts or product can be moved manually with the ESTOP on. The PLC can then keep track of what’s going on and you can start back up again without a problem.
In the semiconductor manufacturing world the rules are you have to kill everything. Be careful about switching the negative. It not only makes trouble shooting a nightmare but in most places its illegal too.
If you have big motors running with frequency drives its safer to keep the power on the drive and allow the drive to decelerate the motors. Don’t put contactors between the frequency drive and the motors. This is a no no and you can damage the frequency drive. Killing the power to a frequency drive with the motor running can also damage the drive.
Remember that the ESTOP is not just for emergencies often the operators will hit the ESTOP just to go to lunch or take a break.
The drawing
I like to use the I/O number for the wire number. You cant always do this because some customers have there own wire number convention but it makes trouble shooing easier when no translation from wire number to I/O point is needed. We say wire number but actually it’s a circuit number. All conductors that are electrically the same are the same circuit number.
Retentive Memory
If you have the PLC keeping track of the machine positions and product on the conveyors all the bits and registers used for that purpose should be in retentive memory. Look at your PLC manual and find out what points are retentive. With retentive points when the power goes down the PLC will maintain the point status and when the power comes back on your machine can start up at the same place in the logic where it was shut down. Otherwise you might have to clear out the machine and start all over again. This can be all right in some cases but if you are moving ten ton rolls of paper it will be a problem.
Safety Circuits
If you have safety circuits for the protection of human life, bodily injury or major machine damage don’t put them in the PLC. There are too many things that can go wrong with a PLC for safety systems. Just going into program mode will disable the circuit. Hard wire all safety circuits outside of the PLC.
Hot backup
You can use a PLC for safety systems with a hot backup. A hot backup is where you have two PLCs in the system with an identical program all powered by a UPS. Then you have a detection system that will swap PLC’s if it detects the first one has failed. This is expensive but if you are using a PLC in a HAZMAT building for detecting gas leaks it’s the way to go.
Startup
Don’t even try starting up until you have checked out all the I/O. Do not load the program. Go through all the inputs one by one and verify the input is working. Have someone operate the device and watch the LED on the PLC. Then mark that circuit as OK on your drawing. Then force all the outputs on one by one and verify that its doing what its suppose to do. The reason for not loading the program is that for some PLC’s an output cannot be forced on if there is a program in there telling the out put to turn off. Take care to mark any changes made to the hard wiring on the drawing.
Doug Hylton
.
I know that its tempting to save some money and hardwire some contacts, selector switches and other stuff between the I/O device and the PLC but resist this temptation.
If you hardwire logic in your I/O wiring the chances are good that during start up you will be out there on the machine pulling wire instead of changing the program. Run everything directly to the devise or sensor. It makes the wiring simple and you will never regret it.
I first started programming PLC’s on pulp bale finishing lines and all of the manual jog buttons were hard wired to the motor starters and only the automatic mode was done by the PLC. The idea was that if the PLC failed you could run the machine with the pushbuttons. I have seen I/O modules fail but I have never seen a PLC fail except for one and that one was connected to 220 Volts and it did smoke.
The problem with only part of the system wired to the PLC was that during start up we always ran into something we couldn’t do because the device was not wired to the PLC.
Sinking or sourcing
I personally like sinking the inputs and sourcing the outputs. I think that sinking inputs are less susceptible to noise. A sourcing input is sitting there low all the time waiting for a high and it could get the high off of a wire laying next to it from a solenoid valve when the field is collapsing. A sinking input is sitting there high and its waiting for a low. I would source the outputs just to make it less confusing. The Japanese like every thing sinking and you see it more in the electronics field.
What happens when you hit the ESTOP
There are two schools of thought here. One is to just kill everything and the other is to leave the processor and inputs powered up and only kill the output power. Just killing the output power has an advantage if the machine parts or product can be moved manually with the ESTOP on. The PLC can then keep track of what’s going on and you can start back up again without a problem.
In the semiconductor manufacturing world the rules are you have to kill everything. Be careful about switching the negative. It not only makes trouble shooting a nightmare but in most places its illegal too.
If you have big motors running with frequency drives its safer to keep the power on the drive and allow the drive to decelerate the motors. Don’t put contactors between the frequency drive and the motors. This is a no no and you can damage the frequency drive. Killing the power to a frequency drive with the motor running can also damage the drive.
Remember that the ESTOP is not just for emergencies often the operators will hit the ESTOP just to go to lunch or take a break.
The drawing
I like to use the I/O number for the wire number. You cant always do this because some customers have there own wire number convention but it makes trouble shooing easier when no translation from wire number to I/O point is needed. We say wire number but actually it’s a circuit number. All conductors that are electrically the same are the same circuit number.
Retentive Memory
If you have the PLC keeping track of the machine positions and product on the conveyors all the bits and registers used for that purpose should be in retentive memory. Look at your PLC manual and find out what points are retentive. With retentive points when the power goes down the PLC will maintain the point status and when the power comes back on your machine can start up at the same place in the logic where it was shut down. Otherwise you might have to clear out the machine and start all over again. This can be all right in some cases but if you are moving ten ton rolls of paper it will be a problem.
Safety Circuits
If you have safety circuits for the protection of human life, bodily injury or major machine damage don’t put them in the PLC. There are too many things that can go wrong with a PLC for safety systems. Just going into program mode will disable the circuit. Hard wire all safety circuits outside of the PLC.
Hot backup
You can use a PLC for safety systems with a hot backup. A hot backup is where you have two PLCs in the system with an identical program all powered by a UPS. Then you have a detection system that will swap PLC’s if it detects the first one has failed. This is expensive but if you are using a PLC in a HAZMAT building for detecting gas leaks it’s the way to go.
Startup
Don’t even try starting up until you have checked out all the I/O. Do not load the program. Go through all the inputs one by one and verify the input is working. Have someone operate the device and watch the LED on the PLC. Then mark that circuit as OK on your drawing. Then force all the outputs on one by one and verify that its doing what its suppose to do. The reason for not loading the program is that for some PLC’s an output cannot be forced on if there is a program in there telling the out put to turn off. Take care to mark any changes made to the hard wiring on the drawing.
Doug Hylton
.
Last edited: