Proper driver interfacing

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mikes
Posts: 94
Joined: Thu Jan 04, 2018 3:09 pm
Acorn CNC Controller: Yes
Allin1DC CNC Controller: No
Oak CNC controller: No
CNC Control System Serial Number: none
DC3IOB: No
CNC11: No
CPU10 or CPU7: No
Location: New Albany, OH

Proper driver interfacing

Post by mikes »

I know there are a number of example of driver interfacing in the schematics and a description of the Acorn I/O in the User's Guide. However, I am finding some of the information confusing. This is probably due to my newness to the subject. Questions:
  • Are the DB25 outputs open collector, with an internal 5v pull-up resistor? The schematic in the User's Guide does not clearly show this (schematics usually have a diamond over a bar to indicate open collector), but the example wiring schematics imply it.
  • I don't understand why a number of the example schematics are using a 5vdc PSU to power the STEP and DIR signals. Almost all drivers use optoisolators for these inputs. As such, a current limit of around 10-18mA should suffice. Wouldn't it be easier, and less costly, to us a current limiting resistor with the 24v supply to achieve the needed interface.
eng199
Posts: 372
Joined: Fri Jan 10, 2014 11:29 am
Acorn CNC Controller: Yes
Allin1DC CNC Controller: Yes
Oak CNC controller: Yes
CNC Control System Serial Number: none
DC3IOB: Yes
CNC12: Yes
CNC11: Yes
CPU10 or CPU7: Yes
Location: Howard, PA

Re: Proper driver interfacing

Post by eng199 »

The DB25 outputs are logic (push-pull) outputs on newer ACORNs (170918+). The screw terminal outputs are open collectors (no pull up).

Page 4 of the user guide shows a triangle for DB25 outputs, which I believe is standard for logic gates. Right next to it is the diagram of a transistor for screw terminal outputs. I don't recall ever seeing the diamond over a bar until just now when I searched for it.

Many of the low budget drives used with ACORN are set up for 5V. Additional limiting resistors could be used for 24V. However, most find it easier to use a 5V supply than splice in additional resistors.

Centroid will carefully consider any suggestions on how to improve documentation.
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