array_devices

Python Interface to currently only the Array 3710A DC Electronic Load This load is also sold under Gossen, Tekpower, Circuit Specialists with same 3710A model number.

All capabilities in programming data sheet are supported.

This has been developed in Python 2.7 32-bit and Python 3.3 32-bit on Windows 7 64-bit. I will try to test on Linux, but don't expect issues.

Possible Program Mode Timing Issues

Note: When using program mode, I have noticed on both of my TekPower 3710A loads that programs run in a little less than 70% of expected time. A 10 second step is completing in just under 7 seconds. I do not plan to use program mode, as I will be scripting the program in Python.

Baud Rate

The only baud rates I can recommend are 9600 and 4800. I have not been able to achieve 100% reliable communication at 19200 baud. 38400 baud often gives me more failures than successes. I have two loads on the same TTL line and running 9600 baud as fast as I can send commands for over 5000 commands without a failure.

My best guess is that it is a consumption data rate issue. Once a load is in failure, if the packet storm is not paused, it will not recover. When I'm talking to multiple loads on a single TTL serial bus, all loads must parse all messages to read if their address is included. So even if the load is spread around multiple loads, the speed of parsing does not change for an individual load. It makes sense to build in some slight pauses between communication to allow recovery points.

pySerial

While not required in the array3710.py code itself, pySerial is required to open the USB serial connection that you provide to initialize the Array3710 object.

Simple Example

See simple_example.py for doing basic interfacing with load.

Programming Load

See programming_example.py for sending a program to the load and running it.

Multiple Loads with Single USB Port

The DB9 cable from the Load to the 3312 TTL Serial to USB adaptor has the following pinout:

1. 3.3V
2. TX (from Computer)
3. RX (from Load)
4. GND

All other pins are not connected.

3.3V power must be supplied from a single load to the 3312 adaptor, because it powers the circuitry on the TTL side of the optoisolators. However, if multiple loads are connected to this 3.3V line, there IS back feed into other loads. If you power up a single load and connect it to another with a standard Y-cable, the micro-controller will power up and begin displaying characters on the LCD. The load will not function and the LCD backlight will be off, because the higher power circuits are not on. This is a potentially damaging situation for the loads.

To daisy chain loads on the TTL side, you need to make a custom splitter. The primary load will provide power for the 3312 adaptor, and MUST be on to communicate to any of the other loads. Pinout for this connection is the full 4 connections:

1. 3.3V
2. TX
3. RX
4. GND

All secondary loads connected should use this:

1. NOT CONNECTED
2. TX
3. RX
4. GND

I only have 2 loads connected in this manner, but you should be able to connect as many as you want, until you reach a noise threshold due to cable lengths. Just make sure to set the addresses to unique values.

Note: I am seeing some backfeeding from the pull up on the signal lines. This is causing the beeper on the secondary load to stay beeping when turned off. This is not a problem if all loads are powered around the same time. The best method to join loads would be multiple optoisolator pairs from one USB to RS232 TLL board.

Talking to Multiple Loads

See multiple_loads_example.py for talking with multiple loads on one USB port.