Can I control the J1970A pod with a modem using the RS-232 port?

The analog side of a modem does not constitute RS-232. Any LAN Analyzer pod can be configured using the "Console" RS-232 port, provided a bona fide RS-232 signal can be provided. Normally, a user would configure a pod using the Window's Hyperterminal accessory. Pods can also be reconfigured (such as new IP) via the RJ-45 port using Telnet. The RS-232 "LAN Analyzer Tap" port is intended for controlling the optional LAN Analyzer 12-tap.