April 2011 IMS 480 Serial card

I finally added the EPROM sockets and supporting components to my Northstar ZPU card.  I inserted the ZPU card with a IMS Model 480 Four Line Asynchronous Communication Controller into my IMSAI.  I added code to the monitor that initializes the serial card and allows character IO.  What I found is the manual description and schematic were slightly off.  I'm pretty sure I purchased the IMS 480 manual from you ten years ago.

If you keep these manuals updated, here is what I found:

 - The on board 4.9152 MHZ XTAL was always connected to pin 16 (XIN) on the 8250 ASIC.  Even more, the 74LS161 @ 14C (sheet 1 of 5) causes the 4.9 MHZ clock to be divided on the output pin 13 (Q1) to around 1.6 MHz.  Thanks to having even this junk 15 MHz scope to let me see that.

- The schematic in Sheet 1 of 5, JB did not reflect my board.  Check continuity past this DIP switch to makes sure ONLY 2 MHZ clock from 8C Pin 18 or on board clock makes it to 16 (XIN) on the 8250.  On my board I actually had to cut (what looked like) a trace between two jumpers for this functionality (as described in the schematic).

- Page 1 of the manual indicates that placing a jumper in the lower pair (4.9152 MHZ) will allow baud rates up to 19.2K baud.  In order for these baud rates, you need to remove the 74LS161 @ 14C.  Jumper pins 2 to 13 in that socket.  Now in your code, use the following equation to get clock divisor:

     8250_DIVISOR = 4915200 / (BAUD_RATE * 16)

 --- Nick Papadonis