Thanks for the reply and the additional information. I placed an order for some parts. If/when I get it working, I will post what I did here to help anyone else who is interested in the same thing.
I have several syringe pumps, a couple of Harvard Apparatus pump 11 models and a few New Era 1000 models. I discovered that a Python library (called “pumpy”) has been created for RS-232 control of Harvard pumps. The NE-1000 pumps can also be controlled using the pyserial Python library for RS-232 communication. The simplest thing would be a completely software approach, which is what I plan to try first. I’ll try to modify the Axidraw control software Python code so that the dropping the pen down simultaneously sends an RS-232 command to start the pump, and lifting the pen stops it. It is possible to program in a pump delay if needed.
An alternative method is to use the B3 pins on the Axidraw board. The NE-1000 pump has a DB9 port on the back that is not used for RS-232 communication. It is instead used to monitor triggers as would be received from a foot pedal controller. Unfortunately, the high/low signal is 5 V, not 3.3 V. The NE-1000 documentation indicates that 3.5 V is the minimum high logic signal. Therefore, I would have to do some logic shifting, or perhaps use a transistor or relay to draw off the 5V pin on the B3 connector to send the high signal.
Digging through the design files for the Axidraw board, it appears that there is zero room for drawing more current from the 5V pin on the B3 connector. It appears that the servo can draw up to 500 mA, and the board has a 500 mA limit for that 5V line. I’m worried that I might overload something. It will be a lot simpler if the Python software pump control works. The only thing needed will be some cables that plug into existing ports.