Absolute parallel encoder to standard DI-card

Someone advised me that it would be no problem to connect an absolute parallel encoder to a standard di-card, because the delay time of the di is only 5 ms and absolute encoders in many cases don't move fast.
Has someone experience with this? If so, how do you do conversion if it is gray-code?
If I want to use a dedicated card (Siemens S7) which one should I use then?

Thanks.

conversion
http://www.plcdev.com/using_ladder_logic_for_gray_code_conversion

sometimes you have to invert all the inputs to get the logic sense correct for the ladder to work

I don't know Siemens, but make sure that if your card is updated asynchronously to the program scan that you could get invalid values without special instructions to avoid have the PLC read a value while the encoder is in the middle of strobing in new data.

I believe with Controllogix, the CPS instruction takes care of that, but I will defer to the Siemens experts to help you decide whether your Input card will be synchronous or not.

With PLC-5 and SLC local Inputs a standard TTL or DC input cards work fine with the MVM (masked move to get only the position bits if less than 16 bit resolution), along with the code examples MichaelG posted.

If the encoder is gray-code, should it be a problem if the card is updated asynchronously?

In OkiePC's example it would still be an issue. He is referring specifically to the Allen-Bradley Logix platform. With this platform the I/O is not updated at the end of the program scan. It is updated based on a time set in the module configuration. This can cause the state of the I/O to change at any point in the program scan. In you case that could mean in the middle of the Gray Code conversion logic.

It's been a while since I used a Siemens PLC product but I believe that they all do I/O updates at the end of a scan. In that case you wouldn't have this problem. But that is something you need to check first.

Keith

In case of asynchronicity, just before the conversion, copy an image of the input bits, if addressable as a complete word into an internal word which can be used bit-by-bit.