Today I tried something different on the 8085, and with good results!!
My first attempt for a software serial port used SID and SOD line for Receive and Transmit signals and bit-bang the serial signals at 2400 baud 8-N-1.
Then I realized that one of the CP/M BDOS calls needed a different implementation. The functions DirectIO and Read Console Status needed the system to receive a char and keep it in a buffer, until the processor asks for it. Reception must be an asynchronous process!
I though about it and without changing much of the previous routine, I connected an interrupt line of the 8085 to an inverted receive line. First I tried the edge triggerd RST7.5 detecting the down flank of the start bit (but then using SID line to get the status and then RST6.5 to detect the start-bit and to read the input line. When an interrupt occurred the serial reception routine would receive the character, store it in a buffer and in a auxiliary memory position store a "buffer full" status. The CP/M function could then be correctly executed.
Instead of using 2 lines (SID and RST7.5), I used only one RST6.5, after the interrupt is received it is possible to "pool" the RST line with the RIM instruction! I save a input line and a few more instructions. It is now apparent to me that the AUX_IN and CONSOLE_IN can both be implemented by software and without using the SID line!
Removing the 8256 added some other problems, I needed a digital output pin to flip the ROM from 0000H to 8000H (and the RAM the other way), for this I added an addressable latch 74LS259.
Have a great weekend.
2 weeks ago