Question about EX_EXPIO.C

JAM2014 · Joined: 24 Apr 2014 Posts: 138

Hi All,

I am starting to write code for a project that will ready a fairly large quantity of digital signals connected to 74LS165 shift registers. There will be 32 '165's for a total of 256 bits that need to be read. I've looked at EX_EXPIO.C and 74165.C, and I think I pretty much understand how the data will be clocked out of the 32 '165 devices. What I'm not exactly clear about is how the multi bytes of data will be arranged. The comments in 74165.C say the code 'clocks bits into the 'ei' structure with ei defined as 'Byte *ei', and that 'acquire_expanded_inputs (&data);' is used to 'read the ei array from the chips'. So, does that mean that the bits are clocked in on 8 bit boundaries (Bytes), and that to access the individual bytes I'd do this:

temtronic · Posted: Fri Jan 13, 2017 7:50 pm

While I've never used the driver, it does mean that 8 bit( 1 byte) is read in at a time, so a complete 74165 is read.
Hopefully someone has played with this code.... when I google 74165, some hits said it'd been discontinued.

One thing to remember is that this is a serial 'chain' so you WILL have to do some 'math' to calculate the acquistion time delay. Worst case is getting data from 74165 #32 in the chain,the one furthest from the PIC. Now if you have 3 extra I/O pins , you can divide the 32 long chain into 2, 16 unit long chains, so access is faster. I did that 30 years ago. There is a trade of of time vs I/O pins

An option for you may be to use PICs as 'smart ,remote I/O' devices. You could easily replace 6 74165( or more) with 3 PICs and only require 3 I/O pins from the master PIC. A 40 pin PIC could have 32 inputs 'muxed' to one I/O line.

Several ways to get 256 inputs,what's best depends upon cost,parts availability,speed of scanning,type of inputs and overall project concept(1 off or 1000 units ?)

Jay

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

You first make an array with the required number of bytes:

JAM2014 · Joined: 24 Apr 2014 Posts: 138

Hi,

Thanks for the clarification! That driver makes complete sense now, and appears to be very easy to use!

As for 'fan-out', the PIC will reside on a 'backplane' board with 4 pluggable 'daughterboards' connected, each with 8 74LS165's. The clock and enable lines on the input to each daughterboard are buffered, so the PIC pins will only be driving 4 'LS buffers.

Thanks,

Jack