How much current can the GPIO ports source/sink? 

The specific source/sink information for each hardware model listed under it's GPIO section of the hardware interface docs pages. For example, see this 

10 nanoseconds is the rise time for our digital signals (rise is the time it takes to go from 10% to 90%

No, the players are digital input and output only.

Is there hardware support for communication protocols? 

The GPIO lines do not support communication protocols. Certain models might possibly be remapped/reconfigured similar to GPIO serial, but it's not a supported function.

Audio, Videos, Images

GPIO

This is our legacy cloud platform. Our latest cloud platform is BSN.Cloud which consists of the free bsn.Control and the paid bsn.Content tiers. Development of BSN has largely ceased and will eventually be deprecated and replaced with BSN.Cloud.

BrightSign_Network_BSN

BrightSign’s *legacy* app for authoring and managing players. The app's actual name is BrightAuthor but is referred to as BrightAuthor Classic to lessen confusion with BrightAuthor:connected, our latest app.

BrightAuthor_Classic

BrightSign’s latest free, all-in-one app for player setup / management, authoring, and content management.

BrightAuthorconnected

The BSN.Cloud record of a player and its Setup. The Default Setup is assigned to the player unless another Setup has been explicitly set.

Provision_Record

The process where a Setup is delivered and applied to a player by BSN.Cloud.

Provision

A set of files containing player settings (player name, description, time zone, Publishing Mode, network settings, etc.) and an autorun.brs file which instructs the player what to do upon bootup.