But if the original was an 18 pin, the new 16F15243 does not come in such a pinout (only 20 pin) and would not be of use without redesigning the board/socket. We are all guessing a bit, as we don't really know the full original part number (type of pins) of the 16C620. The 16C620 PDIP came in 18 pin, the SSOP was 20 pin, but pinouts of the 20 pin are way different than the 16F15243Microchip suggests 16F15243 as the replacement.