USB interface for Keithley 616 electrometer

[nitromethane]

Right, I just finished a new project, that I want to share with you people and the world.

As the title suggests, it's an isolated, Arduino based interface board, that connects to a Keithley 616 electrometer to more or less anything with USB host capability and serial over USB capable drivers.
The circuit board is designed to be built in to the instrument itself, fitting neatly on the existing pin headers on the instrument main board, with the USB connector accessible on the rear panel.
Main source of information for this project was the excellent K616 manual.

Source files are available on github: https://github.com/an-ven/K616-USB-Interface

All input and output signals from the instrument circuitry are electrically isolated from the Arduino with a bunch of optocouplers except for the range switch position signals, which are already isolated from the rest of the instrument.

The interface works by using one of the hardware counters on the Arduino Micro (ATmega32u4) to count pulses of the "gated clock" signal. Number of pulses counted while the "count now" signal is low is equal to the number displayed on the instrument display (ignoring the decimal point).
Other instrument state information such as range switch position, decimal point position and measured value sign are represented on a number of parallel signals, which are also read by the Arduino (barely has enough IO pins).
The Arduino takes those inputs and based on them generates human and computer passable output strings which get sent out over USB serial connection (Two output modes, see README file on github).
Instrument functions zero check, display hold and manual sensitivity selection can also be remotely controlled by sending simple commands back to the interface via USB serial connection.

Comments and questions are welcome and I hope, someone else also finds this useful.

[coromonadalix]

 

[alm]

Nice project! Did you look at how Keithley's digital interface (BCD) model 6162 connected to the 616?

[nitromethane]

Yes I did. My circuit uses the exact same interface that is used for the model 6162. The optocoupler part of my circuit is quite similar to the model 6162, but the rest of the logic circuitry got replaced by an Arduino and some software.