APF Mark 55  
Written by Accutron on 2018-07-22  

Although RPN is primarily associated with Hewlett-Packard calculators, they were certainly not the only manufacturer to produce such devices. Mostek, the primary IC manufacturer for HP's early handheld scientific calculators, also developed the MK50075, an OEM RPN single-chip calculator with both scientific and financial ROM options. This chipset was utilized by several manufacturers, including APF, Corvus, Omron, Privileg and Sanyo.

APF's implementation of the MK50075 is the Mark 55. The Mark 55 is a high performance scientific calculator with a feature set comparable to a HP-45 with added hyperbolic functions. Introduced in 1977, the Mark 55 arrived four years after the HP-45, albeit with a MSRP of only $80.

In addition to the MK50075, the Mark 55 chipset includes the MK50103 and MK50104 scientific ROMs. The financial chipset eschews the scientific ROMs in favor of the MK50101 and MK50102 financial ROMs. While the scientific chipset saw fairly wide adoption, the financial chipset is known only from the Corvus 600, introduced in late 1975.

APF Mark 55 RPN scientific calculator.

Mark 55 calculator, enlargement of LED display.

This article is part of the [Electronic Calculators] exhibit.

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