Digital Input specs?

[mwester]

Sorry if this is a FAQ - if so, then I certainly don't know what terms to search on in order to find this information!
 
I'm in need of the specs for the webcontrol digital inputs (hardware version 2.2.2 if that matters) -- for example, is there a pull-up (or pull-down) resistor, and if so, what value?  Any input capacitors to consider?  Input current?
 
I notice there are two octal buffer chips on the board, one on top, one on the bottom -- I expect that one must be for the digital output; is the other buffering the digital inputs?
 
Thanks!

 

[RAL]

From section 4.1 of the user's manual:
 
4.1 Digital Inputs
WebControlTM has eight digital TTL inputs, each of which can be configured to be
inverted upon input to the system. The PLC engine will then look for a true of false
case of the input. TTL level 0 is defined 0-1.75V; TTL level 1 is defined as 3-5V.
When TTL1 being used as counter/frequency measurement purpose, its counts
pulses up to 2MHz and max count is 32bit number. Counter starts counts from rising
edge after the falling edge.
 
The manual doesn't say what family of TTL is being used, but you can probably assume the input current will be very small.
Something less than 1mA would be my guess.

 

[mwester]

Yeah, that's all I got from the manual as well.  And a simple test with a meter shows that the input current is well under 1ma.
 
But, that's only for simple DC -- I'm in need of enough information about the input to determine how it might affect an existing signal -- for that I need to know the AC characteristics.  A circuit diagram showing any discrete resistors and capacitors on the input lines, and either the specs for the device to which the input is connected (CPU or buffer), or if someone can just tell me what device the input is connected to I can look up the device specs myself.
 
In a nutshell, I need to monitor a set of signals that should remain nice square waves even with the added load of the webcontrol inputs.  I'm uncomfortable with just attaching an unknown device input to those high-impedance lines, even if I can put a scope on the signal lines and crank up the power to compensate for whatever mysterious input the webcontrol might have -- that just feels like such a "hacky" way for me to design a reliable circuit.  I could, of course, design my own octal buffer for the input -- but if I have to design custom input circuitry, I might as well build my own controller using an Arduino or similar.

 

[rossw]

I'm in need of enough information about the input to determine how it might affect an existing signal

 
If your source is that sensitive you really need to be buffering it anyway.
The webcontrol does have pulldown resistors (a disconnected, non-inverted input "floats" low).

 

[CAI_Support]

TTL input has 74LV4245 buffer IC, in its input, there is a 10K pull down resistor for each input. 
The TTL input buffer IC and pull down resistors are on the back side of the PCB, so that they are easy to replace or remove.

 

[mwester]

Thanks for the details -- that certainly clears things up for me.
 
Unfortunately, I'd have much preferred the more-common pull-up resistors, since the drivers for the signals I need to tap into are open-collector.
 
Since I'd rather not hack up the webcontrol board, I'll find a project more suitable for it, and I'll just use an Arduino for this project -- it'll be a lot more coding to make the Arduino do what I need, but it'll be a lot less hardware to build (and maintain).

 

[CAI_Support]

Since WebControl normal TTL logic 1 is 5V, the hardware design must use pull-down instead of pull-up.  If you look the trace on the back of the board, R14 is connected to ground terminals.  You can easily cut that trace and remove the pull-down or wire it to 5V for pull-up.  But then you probably have to invert input in the IO configuration screen.