In 1976 a team of RCA engineers led by Jerry Herzog released their second microprocessor the CDP1802. It used RCA’s CMOS process called COSMOS (Complementary Silicon/Medial-oxide Semiconductor). The CPD1802 was the single chip successor to the earlier two chip 1801 design. Like the 1801 the 1802 incorporated the COSMAC (Complementary Symmetry Monolithic Array Computer) architecture developed by Joseph Weisbecker in the early 1970s. 

One of the items I have had on my bucket list for many years now is creating a complete computer design based around the RCA CDP1802 microprocessor.  My vision, to create an 1802 based computer system with the look and feel found in the vintage computers in the 70s. Something similar to the original Altair 8800 released in the mid 70s. These early computers used simple lights to show the current memory address, output data and to monitor system status. You controlled the system using toggle switches for system control, address and data inputs. 

Back in “the good old days” you sat in front of your system and manually toggled in programs via the toggle switches and hopped to get the results you were looking for on the many lights.

In the end I ended up creating a two board solution.  The first board is the main CPU. It contained everything needed to run as a standalone unit. The second board contained all the electronics needed to create an optional front panel for user interface. The front panel would allow the user to program, monitor and control the operation of the system like days of old.

Don't want a front panel, no problems, here is the turnkey version of the Retro Elf. This Elf will boot up in to a ROM monitor that receives user inputs via a VT100 terminal connected to the DB9 supporting COM1. From the monitor you can do basic debugging or BOOT to Elf/OS. From Elf/OS you can run RCBASIC, SuperBASIC or any number of Elf/OS applications.

I created a 3D printed case to hold the Retro Elf stack of boards, power supply and the three DB9 serial connections. The case was designed to allow the use of two acrylic panels to be slid in to the top and bottom of the case. All the debugging LEDs are viable though cutouts in the sides or through the clear acrylic panel at the top of the case.

Only the front panel PC board is missing from the turnkey unit. The board stack contains the RetroElf2 64K CPU board, the expansion board and the ATA/IDE storage board.

As several Retro Elf builders have noted, the Retro Elf pictured above includes a IDE to CF interface module that is plugged in to the 40-pin IDE ATA-3 connector. This has generated several emails requesting details.  I will try and answer the three most common questions I have received.

Given that the above connector has long straight pins, the 40-pins need to be bent so that one row of 20-pins will be at 90 degrees close to the connector housing. The second row of 20-pins also need to be bent to 90 degrees but care needs to be taken so the resulting spacing between the two rows of 20-pins is about 0.1 inches.

As seen in the included picture above, I used a piece of 0.1 x 0.1 hole spacing prototype board as a tool to help me bend and space the pins. I cut the board so that at least 20 holes on the edge of the board were cut through the middle of that row of holes.

First just use the tool board to help simply bend the first row of 20-pins to 90 degrees close to the connector's housing.

Next place the 20-pins of the second row needing to be bent in the row of hole just above the ones that where cut through on the edge. Bend the second row of pins until the first row of pins come up and inserts in to the half cut holes as shown.

This should result in two rows of 20-pin with a 0.1 spacing. Trim the first row of pins as need to match the second shorter row of pins just bent. Now plug the newly created adapter in to the IDE-CF card's

connector and that assembly in to the IDE on the expansion board.

The OS I have been integrating in to my Retro Elf systems is Mike Riley's Elf/OS. Details can be found at Mike's web site at http://www.elf-emulation.com/elfos.html.

While the Elf/OS works, in my opinion there is still a lot more work to be done. Mike has not done much with Elf/OS since 2008.  I patched both the RCBasic and RC Elf/OS to work with the Retro Elf. There still is room for some software guru types to update and enhance what is there if anyone is interested in taking on such a project.

The backside also provides ventilation holes and fan support to help keep the electronics inside the unit tuning cool.

From the beginning, there was a plan to develop an expansion board. This board would add extra features to the original Retro Elf design.

One of my major requirements was the inclusion of a hardware serial port with handshake. This port needed to support higher baud rates for faster screen refreshes and quicker file transfers. The 16550 UART integrated circuit became the heart of the high speed serial port. In the end, I ended up designing in two 16550 UARTs on to the expansion board.

The serial ports also needed to support the standard RS-232 signaling levels. This was achieved using the MAX232 interface integrated circuit.

To help with serial port debugging and keeping with my many cool blinking lights theme, each serial port included four LEDs that monitors the data input/output lines as well as the two supported in/out handshake lines.

As seen in the above picture, I am adding a battery backed up Epson RTC-72421 Real Time Clock module to the design. This RTC module will gives the Retro Elf a hardware time and date clock feature.

A 40-pin, 8-bit IDE header is also provided to allow the interfacing of external storage devices.

Like days of old, a four pin header is provided to support an external cassette tape storage system. Given the number of tape interface systems there where in the past, the four pin header only brings out +5 volts DC, a driven output on the 1802's Q line and a jumper selectable input to one of the 1802's EF inputs. So some form of appropriate external tape interface will be required to meet your specific needs. 

While the power needs for the expansion board can supplied by only the CPU boards voltage regulator circuit, I also added a second voltage regulator section on the expansion board.  Four jumpers allow you to configure the expansion board to be powered by the the CPU's regulator or run with it's on board voltage regulator.  It is even possible to power the Retro Elf's front panel via either the regulator circuits.

As mentioned earlier, one of the interesting features of the Retro Elf's CMOS construction is the ability to adjust the clock speed at any time to any rate. Here we see the first bread board stage for the Slow Clock option designed to take advantage of this.

This system is NOT a single step operation where the 1802 clocks a full speed through fetch and execute and then waits at the end of the cycle for you to say another step please.

A slow clock feature is a great way to both debugging hardware and software.  It also allows you to see how the 1802 process code.  You will discover that the address LEDs no only display the address of the next memory operation but also the 16 bit data being moved from internal registers. So a decrements of an internal register will show up on the address LEDs during the execute phase and then return to the next memory location on the next fetch.

I have used the above prototype clocking systems in classes to show students the inner workings of their code. For me, it is exciting to see the lights go on in the student head as the LED lights reveal what going on inside the Elf as it step through their test code.

Originally I was going to use a 555 timer IC but ultimately went with a dividing of the system clock.  The resulting design uses a three position toggle switch with the same look a feel as the toggle switches already in place on the front panel of the Retro Elf. In the up position, you get full clock speed normally 3Mhz.  In the second, middle position, the system clock is divided down to ~183Hz. With the toggle set in the down position you get the lowest speed of ~3Hz.

Testing showed that the middle speed was just fast enough to push through code but still get some useful indications from the status LEDs.  The slowest speed gave you the time to really watch each clock cycle happen.

In reality you can use jumpers to select one of four frequencies for both the middle and down positions. If you wish, you can reverse the order so that the middle position is very slow and the bottom is the faster speed. It is even possible to make both middle and bottom the same clock speed! The following table shows each of the four possible setting when a 3MHz master clock is used:

Jumper	183Hz	46Hz	11Hz	3Hz
A0	Out	In	Out	In
A1	Out	Out	In	In

Another idea I have been working on is the addition of a Shadow ROM circuit. The above CPU board is the first attempt of an implementation of this concept.

The Shadow ROM concept is simple. When the system is first booted, the ROM monitor is running from the on-board ROM. The ROM monitor does any power on self tests needed and then copies itself in to RAM that is addresses in the same physical location as the ROM.

This magic is accomplished by allowing any memory reads to come from the ROM after boot up and any writes will go only to the RAM located at that same address range. Copying is then a simple matter of read the ROM byte from the current memory location and writing that byte back to RAM again the same memory location.

Once you are done coping any needed code from the ROM in to RAM, you then write to a specified output port that disables the ROM requiring all future reads and writes to come for the RAM.

The nice feature of having Shadow ROM is once you turn off the ROM you get a full 64K or RAM to use with any running applications in memory.

There is a down side to the above implementation, there is no power-on booting. You need to use the Retro Elf's front panel to point to the location of any ROM monitor code to be executed.