Dynomotion

Group: DynoMotion Message: 12332 From: collimero Date: 10/1/2015
Subject: connecting limit switches

Hello Tom,


I have a question concerning physically connecting limit switches to a KFlop. What I want to do is use the Gen Purpose LVTTL I/O's 0 to 7 to read out the status of several switches or relais.  I'm not an electrical engineer, so I wonder if it safe to connect a switch to the 5V output of the board and directly connect the other side of the switch to the corresponding pin (any pin between 7-14 on JP7). Or would it be safer (or even nescessary) to add some additional external resistance (or circuit) between switch and input pin to prevent too high currents throught the ports?


I can imagine that in case an IO port is directly connected to 5V and it is accidently configured as an output and set to 0, this could easily damage the port. So, in a more general sense I wonder about what kind of protection is present on the ports of the KFlop board to prevent them from geting damaged accidently, and what protection is required from external hardware.


Best regards, 

Jeroen 

     

 
Group: DynoMotion Message: 12333 From: Tom Kerekes Date: 10/1/2015
Subject: Re: connecting limit switches
Hi Jeroen,

The KFLOP LVTTL I/Os are 3.3V signals diode clamped to the 3.3V power rail.   They should not be driven above 3.8V or may be damaged.  Most +5V TTL outputs do not do this as they are only required to drive 2.8V or higher. The KFLOP inputs marked 5V Tolerant have series 47ohm resistors to prevent damage if driven to 5V.  But this should be avoided.

If you wish to use a switch to drive the inputs high they may be switched to 3.3V.  Of course opening a switch will not guarantee that the inputs go low as they may still float high.  A pull down resistor should be used to guarantee the input goes low.  The first 8 inputs on KFLOP JP4 and JP6 have 150ohm pull down resistors that may be useful for this purpose. 

The I/O should not be configured as an output and at the same time driven externally.  The output drive is internally limited to ~16ma so it usually doesn't cause damage, but regardless you should never do this.

HTH
Regards
TK

Group: DynoMotion Message: 12335 From: collimero Date: 10/2/2015
Subject: Re: connecting limit switches
Thanks Tom, this info cleared up things for me. I will add some pull-down resistance to the limit switches and will use 3.3 volts.
Best regards, 
Jeroen