Intel SDK-85 MAX232 Modification

The Intel MCS-85 System Design Kit has a serial interface, but since it was introduced in the late 1970's it runs at 110 baud and on a current loop. The user manual for the kit describes how to convert this to a more modern RS-232 interface, but using the somewhat obsolete MC1488/MC1489 chip pair which in this case require -10 volts.

A better replacement for MC1488/MC1489 is the MAX232 which is a single chip that can run on just +5 volts. The serial interface directly on the Intel 8085 is TTL-level so I first thought about using a common TTL-level UART to USB converter, but none that I could find supports baud rates down to 110, only 300. The traditional 8250-based UARTs found on older PCs can run fine on 110 baud so I can use that to talk to the MAX232 using regular RS-232 signalling.

One additional challenge is that the transmit signal from the Intel 8085 needs to be inverted before being sent to the MAX232, but only the transmit signal and not the receive signal! I used a common 74HCT14 inverter with Schmitt-trigger for this purpose.

Here is a photo of the modification:

A DE-9 D-Sub breakout is used to complete the connection:

Here is a how the different transmit signal levels should look like:

1) From the Intel 8085.
2) Inverted through the 74HCT14.
3) Level-shifted through the MAX232.

When using Linux on a PC with a null-modem cable to the MCS-85 SDK, the "cu" command can then be used to connect to the serial interface like this:

cu -l /dev/ttyS0 -s 110 --parity=none
          

Here is a schematic of the modification: