A fluid supply cartridge vertically insertable into a fluid-ejection device, comprising: a housing; a supply of fluid within the housing; a digital fluid level sensor within the housing and in contact with the fluid to measure a level of the fluid within the housing; and a vertical interface at a surface of the housing to connect the fluid supply cartridge to a fluid-ejection device, the vertical interface comprising: a fluid interconnect septum to vertically fluidically interconnect the supply of fluid to the fluid-ejection device; and an electrical interface to conductively connect the digital fluid level sensor to a corresponding electrical interface of the fluid-ejection device; wherein the fluidic interconnect septum is a first fluidic interconnect septum to supply the fluid of the fluid supply cartridge to the fluid-ejection device, and wherein the vertical interface further comprises a second fluidic interconnect septum to return the fluid and air from the fluid-ejection device to the fluid supply cartridge. a housing; a supply of fluid within the housing; a digital fluid level sensor within the housing and in contact with the fluid to measure a level of the fluid within the housing; and a vertical interface at a surface of the housing to connect the fluid supply cartridge to a fluid-ejection device, the vertical interface comprising: a fluid interconnect septum to vertically fluidically interconnect the supply of fluid to the fluid-ejection device; and an electrical interface to conductively connect the digital fluid level sensor to a corresponding electrical interface of the fluid-ejection device; wherein the fluidic interconnect septum is a first fluidic interconnect septum to supply the fluid of the fluid supply cartridge to the fluid-ejection device, and wherein the vertical interface further comprises a second fluidic interconnect septum to return the fluid and air from the fluid-ejection device to the fluid supply cartridge. a digital fluid level sensor within the housing and in contact with the fluid to measure a level of the fluid within the housing; and a vertical interface at a surface of the housing to connect the fluid supply cartridge to a fluid-ejection device, the vertical interface comprising: a fluid interconnect septum to vertically fluidically interconnect the supply of fluid to the fluid-ejection device; and an electrical interface to conductively connect the digital fluid level sensor to a corresponding electrical interface of the fluid-ejection device; wherein the fluidic interconnect septum is a first fluidic interconnect septum to supply the fluid of the fluid supply cartridge to the fluid-ejection device, a fluid interconnect septum to vertically fluidically interconnect the supply of fluid to the fluid-ejection device; and an electrical interface to conductively connect the digital fluid level sensor to a corresponding electrical interface of the fluid-ejection device; wherein the fluidic interconnect septum is a first fluidic interconnect septum to supply the fluid of the fluid supply cartridge to the fluid-ejection device, and wherein the vertical interface further comprises a second fluidic interconnect septum to return the fluid and air from the fluid-ejection device to the fluid supply cartridge.