To Your Basket Ordering Information Part Number: R297-SD84 Price: $213.00 Weight: 0.10 lbs The SD84 is a USB interfaced controller for up to 84 servos, which maintains a 20ms refresh rate irregardless of number of servos or servo positions. Each of the 84 channels can be set to Digital Input, Digital Output or Servo Output. Additionally, 36 of the channels can act as 10-bit Analogue inputs, making this a great animatronics controller.

More Details:

• Up to 84 channels of digital input, digital output, and servo output
• Up to 36 channels of 10-bit analog input
• Refresh Rate - 20ms under all conditions
• Control and Power Interface - USB
• Drivers available for Windows, Apple, Linux and Open BSD
• Servo Power - separate terminals for voltage of your choice

A schematic of the SD84 can be found here.
The SD84 has 84 sets of 3-pin headers, in groups of 4 at a time. In all cases, the pin nearest the outside of the PCB is 0v Ground. The middle pin is +V power for the servo from the associated 2-pin screw terminals. The inside pin, nearest the CPUs is the signal pin.

Internal Registers
There are three internal registers associated with each of the 84 channels.
A "Mode" register (1 Byte) that defines the current mode of the channel, INPUT, OUTPUT_LOW, OUTPUT_HIGH, SERVO or ANALOGUE.
A "Servo" register (2 Bytes) that defines the position of the servo in μs. The "Servo" register is an integer (two bytes). The speed and low byte/high byte of the position.
A "Speed" register (1 Byte) that defines the speed of the servos while changing position.

Related Links:

Comparing and Explaining the Acroname Servos