Basically, the PB-300 was a PB-100 with more RAM and a built-in
thermal printer. To our knowledge, Casio was the only company who
designed real pocket size computers with integrated printers.
Of course, the PB-300 was substantially larger than its elder brother, therefore the keyboard could be laid out more generously, with a real space bar and an additional key for paper feed.
Due to the different physical dimensions, the main PCB had a different layout.
However, the logic circuits were identical to the PB-100, with an additional
HD61914 RAM chip. No further RAM expansion was made available.
As printers needed much more energy than the computer components, a 4 x 1.2V NiCad battery pack had to be integrated, which needed to be charged with an external power adaptor. The necessary printer and battery charging circuits were built onto a second PCB.
The printer used thermal paper of 39 mm width and was able to print 20 characters per line, making printed listings a little bit more legible than on the 12-character display.
To sum up, the PB-300 was a really cute machine. Imagine taking it with you in 1983 and impressing your friends by printing business cards when needed.
The PB-300 was also sold in Europe under the name Olympia OP-644.
Thanks to Roman von Wartburg (retrocomputing.ch) for information and pictures.