The front panel interface is not a serial or parallel interface in the usual sense. In fact, it is a shared memory area that can be accessed asynchronously by both the main assembly processor and the front panel processor.
As far as I know, the first 47 bytes of external memory on the H8 MCU side are a serial buffer of character codes for the 1st and 2nd lines of the display. The next 18 bytes are the annunciators flags (0x00 - off, 0x01 - on).
But besides this, the same memory contains a keyboard buffer, a matrix of pressed key flags, an interrupt messaging mailbox, and a large number of flags, the purpose of which I do not know (most likely they are related to scrolling, blinking, etc.).
The H8 MCU firmware contains a character generator with the standard ASCII character set, however, I found that characters with codes 0xE0...0xE8 and 0x7F are used in debug mode to smoothly fill the display with vertical and horizontal lines.