;EP - wait for the host to initiate communications
; usually, this is an ENQ
; sometimes, it may be ACK/ENQ or ENQ/ACK (Envoy 4.1 page 6)
; Returns 0 if success, error code if failure
; Error code 30101 - disconnected
; This probably means that the host system only gives us one
; transaction per phone call, and we were hoping for an ENQ to
; let us send a second transaction, but the host sent EOT instead.
; Error code 30102 - nothing received and we hung up