Return of the elf [Resources_Hands On]

A couple of months ago, I built the Membership Card, a remake of the 1976 Cosmac Elf microcomputer. Despite the vintage of its RCA CDP1802 processor, the Membership Card still has value as a low-power microcontroller, with an elegant instruction set that leverages a clever hardware design. However, only a masochist would attempt to do any serious programming with the Membership Kit alone: Entering a program via the Membership Card???s front panel requires using toggle switches to enter bytes into memory, one bit at a time. · What´s needed is a way to upload programs written with the aid of those sops to human frailty, keyboards and screens. There are actually a number of ways to get such programs into the Membership Card, which is composed of one circuit board that is a complete microcomputer, with processor and memory, and another board stacked above it, which is the front panel that provides general input/output facilities. One way is to burn a program directly into an EEPROM chip and mount it on the microcomputer board. A more fl exible option is to burn a small loader program onto an EEPROM and then upload programs as desired via a serial connection. · To be sure, this is probably the best approach if you intend to use the Membership Card with shields created for the Arduino, for example. Bill Rowe has created a replacement board for the front panel???the Olduino???that allows exactly this, supplying an interface for modern shields that provide things like Ethernet connectivity. · But these options require hardware modifi cations. Instead, I wanted to use the parallel port interface built into the existing front panel. With this I could build a programmer that would???electronically speaking???act · like a human being flipping switches and entering bytes, albeit a fast and error-free one.