drogon wrote:
Maybe just stick an FPGA/CPLD on the carrier and make it 6551 compatible without the bug? This has been done for much more complex stuff in the past - e.g. the TI99 graphics chip thingy... Anyone up for the challenge?
Using a naval analogy, making an FPGA emulate a 6551 seems too much like dispatching a battleship to sink a rowboat. A PLCC44-to-DIP adapter is a whole lot less work.
In the case of the NXP 28L92, two pins are no-connects, so the adapter could be designed around a
42 pin DIP socket. The logical way to construct the unit would be to directly solder the UART to the board so as the eliminate the cost of a PLCC44 socket. PLCC uses 50 mil J-leads, which can be manually soldered.
cbmeeks wrote:
Could even have a place for RS-232 level converters.
That could be done. However, not all applications would need TIA-232 voltages, so you'd have two different designs to accommodate those who want TTL connections vs. those who want TIA-232. Better to have the TIA-232 transceivers on the mainboard instead of the UART adapter.
Getting back to the 6551 itself, it's a very obsolete design that reflects 1970s technology. About all it has going for it is it's easy to interface to a 65xx bus and it has a very simple, albeit brain-dead, programming model. Much better UARTs are available.