PBJ Wordpak/Wordpak II

This cartridge provides monochrome video out to a monitor connected directly off the cartridge. It patched both extended and disk basic to work with its video output; it had a bootable eeprom (WordPak I only) for patching extended basic and bin images on disk for patching disk basic. The default software provided up to 80x24 of text, much more than the CoCo 1 or 2 was capable of natively. This was helpful for text-heavy applications such as word processing and programming. Additionally, one could create custom fonts using a 8x8 or 8x10 (for the Wordpak II) pixel matrix. One could also set other video modes - the 64 character wide screen was good for porting TRS-80 Model 1 programs. Thought not supported by the provided software, the chip is capable of even higher resolutions than 80x24 if one has a capable monitor.

The Basic patches provides a screen editor, some additional keyboard functions, and control codes.

The cartridge has a SY6845E chip (a extended clone of the MC6845, itself a clone of the HD46505), some onboard RAM, a character generator, and some addressing and latching logic.

The cartridge is operated via the following memory-mapped registers:

• $FF98 read as a status register, or written to select the register for the 6845 chip.
• $FF99 to write a value to the selected register
• $FF9B is read or written as the display memory latch (after setting registers 18 and 19 appropriately then selecting register 31).
• $FF9C (WordPak II only) could switch between the WordPak II output (poke 64 to this location) or the computer output (poke 0 to this location). The WordPak II had an input plug so one could route the normal video out through the cartridge giving this software switch control, so one didn't have to swap cables by hand if one had a single monitor. This was more useful if one modified the CoCo to output composite video instead of RF.

See manual linked below for register list and typical initialization. Registers are used to modify resolution, cursor types, etc.

Implementation detail: In an integrated system design, the 6845 chip could share memory with the system processor (much like the CoCo 1 and 2's onboard MC6847 video chip). In such a design, the processor could write directly to a shared memory area that is also read by the video chip, making for very fast and efficient screen updates. However, shared access from both the processor and video chip requires synchronization logic (which is what the CoCo's SAM chip does). However, the expansion port doesn't provide the necessary connections to extend this capability into a cartridge. Fortunately, the SY6845E chip (as opposed to the MC6845 it was based on) also has a "Transparent Address Mode" that allows writing to separate dedicated video memory. This made for a much simpler interface doable with a cartridge, but also meant the software was more complex: changing a single character on screen required 5 writes, a status read, and then the final write. The chip did have auto increment logic that reduced the writes when writing sequential data. The 6845 was used on several systems, including the original IBM PC (including the MDA, HCG, and CGA adapters), the BBC Micro, and the Amstrad CPC.

Links:

• Manual
• 1988 Radio Shack Catalog Entry
• SY6845E Datasheet
• Teardown and build your own clone