Connecting to Digital Ouputs on T100MD-888+

[Kevin Li]

Hello,

I am new to the PLC and electronics world.  So please bear with me.

My question is concerning the hookup of the T100MD-888+ PLC to a motor driver (MD2S-P)
http://www.usdigital.com/products/md2s/

In the MD series Installation manual it suggests to use a current limiting resistor to make the PLC compatible with the motor driver's 5V inputs.
I did the math as per Page 9 of the manual (http://www.tri-plc.com/MD-man.pdf)

It is my understanding that this is because when the PLC is logic low, it pulls up to +24V.  So the stepper driver would experience a very large current flow if it was not restricted by a resistor.

Calculations:
Given I_f = 5.5mA
(24-5)/0.0055 = 3454 Ohm.

So all that I would need is a 3.6k to 3.8k resistor for each line to limit the logic low current from the PLC to the stepper?

Also, is this "opto-isolated" as they suggest as necessary?

Any help would be greatly appreciated.

Cheers,
Kevin Li

[support]

Yes, this driver has isolated inputs.

1) connect isolator ground to PLC ground.

2) connect PLC's 24V power supply to the "Isolator Power" input.

3) Connect STEP in series with a 3.7K ohm resistor to the PLC output #5.

4) Connect DIRECTION in series with a 3.7K ohm resistor to the PLC output #1.

5) If you use the PLC output to control the "Reduce Current" and "Inhibit" inputs, then also connect 3.7K ohm resistor to each.

Run the STEPMOVE command to run the motor.

[Kevin Li]

Hello,

Thanks for the reply.  Saves me having to buy a 5V power supply.

Would I also need current limiting resistors on the connections between:

"Step to PLC Output #5" and  "Direction to PLC Output #1"  ?

It recommends in MD-man.pdf (section 1-3 of http://www.tri-plc.com/MD-man.pdf) to add resistors between the PLC and step/direction so that the logic low current from the PLC's to the Driver is below the maximum allowed (5.5 mA).

ie. R = (24-5)/(max low logic input current) = 19/0.0055 = 3454 ohm.  Choose 3.7K ohm.

So I would put 3.7k ohm resistors between step to plc output #5 and direction to plc output #1.

Thanks for the quick response! I really appreciate it.

Kevin

[support]

Sorry, the 3.7K ohm resistor is not to be added to the "isolator power" input. Instead, one 3.7K ohm resistor each is to be added in series to the STEP and DIRECTION inputs to the driver as suggested in the manual.  

The 24V DC can be connected directly to the "Isolator Power" input as long as you do remember to add the 3.7K to each of the 4 interface input to the stepper driver (including the "Reduce Current" and "Inhibit" input if you use these two inputs).

Alternatively, you can also use a 3-terminal 7805 to convert the 24V DC power to 5V and connect the 5V to "Isolator power" input.  Then there is no need for resistors and you can follow the wiring shown in the stepper driver's installation diagram. Since the total current is low (5.5mA x 4 = 22mA) a 7805 can be used without heatsink and is low cost and easy to use (one terminal to 24V, center leg to 0V and the other leg is 5V out).