SX is the generic name for the Scenix SX18 and SX28, which are almost the same. For a jal programmer the only difference is that the SX18 comes in an 18 pin DIL package and the SX28 comes in an 28 pin (slimline) DIL package. Due to the smaller package the SX18 does not provide pins for the C port, hence the SX18 has 12 general-purpose IO pins and the SX28 has 20. There is also an SX20 which is just an SX18 with two extra unconnected pins.
The SX is a clone of the 12 bit PIC architecture (pic 16c54 etc.) with some (configurable) extensions. JAL always configures the SX for an 8-level stack, single-cycle execution (turbo mode) and no carry in for the arithmetic instructions.
The SX is a single-chip microcomputer in an 18 or 28 pin package. 12 or 20 pins are available for general-purpose I/O. The chip has 2K of electrically (re-) programmable program memory, 128 bytes of RAM (called registers in Microchip/Scenix terms) and an 8-level return stack. There is no simple way to expand any of these on-chip resources. Most instructions take one clock cycle, so the peak CPU horsepower is 50MIPS at the maximum clock frequency of 50MHz. However the instructions which change the PC (goto, call, return, btfss, btfsc and instructions which manipulate the PC directly) take three clock cycles, and the banked (register) and paged (code) architecture requires a lot of extra instructions so direct comparisons between the SX and the 16x84 (14 bit PIC architecture) requires some care.