Overview
The Epson HX-20 was one of the first examples of a laptop form factor portable computer from the early 1980s. They were an interesting machine with a good keyboard, a microcassette drive that plugged into the main unit and an optional 16K RAM Expansion Unit.
Given the available choices in low power SRAM chips back in the day the 16K RAM Expansion Unit was rather large. As an alternative a much smaller modern unit has been created which plugs into the same expansion port as the original but is only about the size of a pack of chewing gum.
Usage and Configuration
Owing to the odd memory map of the HX-20 the internal OPTROM overlaps half of the memory range available for the 16K RAM expansion. If you are using an OPTROM like SkiWriter, the RAM expansion can be set to 8K by means of a switch on the side.
Setting the switch toward the dot will allow the OPTROM to still work and provide 8K of additional RAM. If the OPTROM is removed from inside the HX-20 the switch can be moved the other direction providing an additional 16K of RAM. You cannot have the switch in the 16K position with an OPTROM installed in the HX-20. The switch is accessible from the side with a tool like a straightened paperclip. It is more easily accessible from the back of the unit before you plug it in, however.
To install the RAM expansion, turn your HX-20 off, set the switch on the expansion module to match your requirements and plug it into the expansion port. Turn on the HX-20 and press Ctrl+@ to enter Initialization mode. Set the date and time as indicated. To check the amount of RAM available go into BASIC and type in ‘PRINT FRE(0)’ and you will get 29,275 with 16K enabled and 21,083 with 8K enabled.
