"BYTE" criticized Novation, stating that "the company's literature should be very explicit" about the modem's inability to communicate at 1200 baud with Bell 212 modems without the expansion card, but concluded that "Apple users shopping for a modem would be wise to consider this system very carefully". Despite the Apple-CAT II's popularity, its early years were faced with incompatibility with most telecommunications programs, as its native API is considered proprietary by today's standards. To increase compatibility without re-coding these programs, Novation released a firmware upgrade kit (an EPROM package) allowing the modem to emulate the Hayes Micromodem II, including the MMII's IN#x control codes. At the Apple-CAT II's peak, compatibility with its native API increased, and MMII emulation became less of a necessity.