Controlling TGY-5521MDHV Servos with Arduino Uno R3

Good morning, everyone!

I'm using the arduino to control servos as part of my attempt the inmoov open source robotics project's prosthetic arm.

I have little experience with servo control, but I have been using the "servo.write" command to control MG946R servos.

I recently decided to switch to TGY-5521MDHV servos and I've been running into problems. The servos are connected to an external 7V power source, and though they respond to voltage being applied, they aren't affected by the commands from the Arduino at all.

The specs online say these servos use an 8ms pulse, as opposed to a 1ms pulse used by the TowerPro servos. Does this affect the code I need to write? Does anyone have experience with these servos and know why they may be locked up?

Thanks!

Did you connect the grounds?