The TRS-80 Model I and III used a character generator (CG) chip to generate the glyphs on the screen. The 2x3 graphical characters were not in the CG chip; these were generated using discrete logic on the fly.
The Model I used a MCM6670 CG chip, providing 5×7 pixels for each glyph.
It's helpful to think of the character set as being broken into 32-character banks. The Model I's CG chip had four banks for the first 128 characters: symbols, digits, upper case, and lower case. The lower case characters were especially bad, with raised descenders and the letter "a" inexplicably raised two pixels:
Video was 1024 characters (16 rows of 64 columns), and the memory for this was split across 1024-kilobit chips, one chip for each data bit. To save money, Radio Shack omitted the chip for bit 6, and its value was hard-wired to return the NOR of bits 5 and 7. The resulting byte was used as the address for the CG chip. This had the effect of duplicating bank 2 (digits) to bank 4, and bank 3 (upper case) to bank 1:
The Level 1 Basic used the ASCII values, meaning bank 2 for digits and bank 3 for upper case letters. Level 2 Basic, however, used bank 1 for upper case letters. This made no difference since banks 1 and 3 looked the same.
The Electric Pencil word processor shipped with an unofficial modification to add a bit 6 memory chip. This allowed the word processor to display the lower case letters, but also caused the Level 2 Basic to display the bank 1 symbols instead of upper case letters. A physical switch was added to the keyboard to revert to the computed bit 6 value when not running the word processor.
Radio Shack later sold an official lower case modification kit (26-1104). This not only added the bit 6 RAM, but also replaced the CG chip to provide better lower case letters. To be compatible with Level 2 Basic, bank 3 was duplicated to bank 1. Note that the CG chip still didn't have space for true descenders, so they raised all lower case non-descender letters up one pixel. Also the backquote was replaced with a pound sign, and the tilde was replaced with a yen sign:
The Model III's CG chip's glyphs were 7×8, allowing lower case letters to sit on the same baseline as the upper case letters. A nice set of Latin letters were placed in bank 1, and the backquote and tilde were restored:
On the Model I, the entire high half of the character set were the 2x3 graphical characters. These only take up 64 slots, but the bit 6 hack forced them to be duplicated (banks 5 and 6 were the same as 7 and 8). On the Model III the graphics characters were placed in banks 5 and 6, leaving another 64 free slots. By default these had Greek, math, and drawing characters:
Clearing bit 3 of port 0xEC switched these to Katakana characters. In
Basic this was toggled by running PRINT CHR$(22):
Both models supported a double-width character set. These were not higher-resolution
glyphs, they were only the above glyphs with each column duplicated. Characters were
stored in the even bytes of video memory; the odd bytes were ignored. Setting
bit 2 of port 0xEC turned on wide mode. In Basic it was enabled by
running PRINT CHR$(23) and disabled by clearing the screen (CLS).
Glen Kleinschmidt's write-up of his Model I clone has lots of great details about the video circuitry of the original. His posts in the EEVblog forums also have great info.
Matthew Reed wrote up the Electic Pencil lower case modification.
Rebecca G. Bettencourt has a wonderful set of TRS-80 fonts.
The fonts in this emulator were taken from the xtrs emulator.