Keyboard with RS485 protocol will control stepper motors by Arduino

Using Arduino Project Guidance
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Dear all,

I am going to develop a pan/tilt unit for CCTV system.
The stepper motors inside the pan/tilt unit will be controlled by Pelco RS485 keyboard (Controllers & Keyboards – Security System Accessories | Pelco).
I use Arduino UNO Board , ST6600 drivers and stepper motors type JK42HS40-1704 for this project.
The pan/tilt work well by a analogue joystick. Now I go further to Pelco RS485 keyboard.
I hope somebody in this forum can give me the idea how to converter RS485 signal to analogue signal or some other idea for my project. Thank you.
Regards, elc

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I hope somebody in this forum can give me the idea how to converter RS485 signal to analogue signal

Suppose that someone presses the "G" key on the keyboard. How does that map to an analog value?