previous series of Saturn-based ICs were codenamed Lewis, after Meriwether Lewis. Common for all models CPU architecture: Saturn Screen resolution: 131×64 pixels. There were 3 versions of the LCD display used across the range with each improving visibility and contrast over the predecessor. Communication ports: 4-pin RS-232 (Serial port, standard UART at 1200 to 9600 bauds with option for parity (none/even/odd/mark/space)) and Infrared port (not IrDA, similar to serial, zeroes are pulses with duration 1/8 of a full bit time) Data bus width: 8-bit (external) Maximum 4-bit-cell address width: 20 bits (leading to the address space shown next) Logical address space: 512 KB Maximum register size: 64 bit (both working and scratch registers) Available expansion card ports on X models: 2 Expansion card pins: 40 HP 48S/HP 48SX specific CPU clock frequency: 2 MHz Memory clock frequency: 2 MHz CPU codename: Clarke (Saturn 1LT8 core) Communication protocol(s): Kermit (protocol) On-board ROM: 256 KB On-board RAM: 32 KB Maximum additional memory per expansion card: 128 KB (48SX only) ROM versions: A, B, C, D, E, (F,) J Orange and blue shift buttons HP 48G/HP 48GX/HP 48G+ specific CPU clock frequency: 3.68 to 4 MHz Some claim that the frequency varies according to temperature According to one of the engineers on the design team of the HP 48G series (Dave Arnett), the yields for 4 MHz CPUs were essentially separated into two bins: the ones closest to spec, generally near 3.93-3.94 MHz, were reserved for the expandable models (GX), and those just slightly under spec were used for the non-expandable units (G). Eventually the yields improved and the CPUs which clocked closer to 4 MHz were installed in the non-expandable units as well. The effects of (non-extreme) temperatures are almost negligible. Memory clock frequency: 2 MHz CPU codename: Yorke (Saturn HP 00048-80063 core) Communication protocol(s): Kermit (protocol), Xmodem On-board ROM: 512 KB On-board RAM: 32 KB (48G) or 128 KB (48G+/48GX) Maximum additional memory for expansion card port 1: 128 KB Maximum additional memory for expansion card port 2: 4 MB (128 KB addressable at any given time via bank switching) ROM versions: K, L, M, P, R Purple and blue-green shift buttons. This is sort of a design error, since these colors may be indistinguishable for red-green color blind people Programming The HP 48 series of calculators support a stack-based programming language named RPL, a supposed combination of Reverse Polish notation