The RPL programming language was not as intuitive as the former RPN. It came with two user's guides, totaling about 675 pages demonstrating the complexity of the calculator. This and the lack of ability to import or export programs limited the popularity of the model for programming development and sharing. On the other hand, its extensive built-in library of math, statistical, conversion and financial functions made it extremely powerful.
The HP-28C had the ability to wirelessly connect to a printer via an infrared transmitter. Shown here (left) is one I found on eBay for not much. This model printer is still sold today to connect to HPs current graphing calculators.
HP enthusiasts were not happy with the move away from the intuitive RPN to RPL. To answer their concern, HP introduced in 1988 the HP-42s. This calculator was compatible with the HP-41C series but offered a more compact package and utilized a unique menu system which allowed to reduce the number of keys and the number of functions assigned to one key. While it had all the functionality of the earlier HP-41C, it lacked connectivity. It could not import or export programs and it could not connect with peripherals.
|Free42 - an HP-42s Emulator for the iPad|