An implantable infusion pump possesses operational functionality that is, at least in part, controlled by software operating in two processor ICs which are configured to perform some different and some duplicate functions. The pump exchanges messages with an external device via telemetry. Each processor controls a different part of the drug infusion mechanism such that both processors must agree on the appropriateness of drug delivery for infusion to occur. Delivery accumulators are incremented and decremented with delivery requests and with deliveries made. When accumulated amounts reach or exceed, quantized deliverable amounts, infusion is made to occur. The accumulators are capable of being incremented by two or more independent types of delivery requests. Operational modes of the infusion device are changed automatically in view of various system errors that are trapped, various system alarm conditions that are detected, and when excess periods of time lapse between pump and external device interactions.