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I have an old peristaltic pump with a 9 pin male sub-d socket labeled Gilson Serial Input Output Channel (GSIOC) that connects a master device to devices with RS422A specifications, or RS485 specifications. The GSIOC implements RS422A/RS485 transmitters and receivers.

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Could any computer serve as a master device by simply connecting the pump and computer directly through a serial to USB converter with proper specifications?

Building from here could serial commands be written on and sent directly from the computer to the pump in order to control speed/rotor direction...etc? Or am I missing important conceptual understanding on how to remotely control the pump?

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Yes that will be feasible.

It appears from the image that the green 6pin connector is for external manual controls. One of the signals is an analog voltage, presumably to control the motor speed.

In the RS232/485 signal there should be a digital PWM (pulse width modulated) value to control the motor speed through a power driver (FET/transistor/SCR/etc).

The other signals serve as on-off commands to manage the motor enable and direction.

So you will need to find which DB9 pin serves as the speed control signal - (Analog V+) and feed a variable PWM source to that pin. Now RS232/485 devices are not high current devices, so you will need to figure out how the power is provided to drive the motor.

Other than that the computer/MCU will be able to set each pin on the DB9 to a LOW (0v) or HIGH (5V or 3.3v) voltage which is then passed through the RS232 "converter" to shift the signals to the correct levels (+/- 5V) to control the device.

Once the wiring and power are known this is an easy job.

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