3.3-to-5 level converter on inputs keeps levels always high

Electronics and hardware
1

Trying to interface my old Sharp PC-1403, with its 11-pin connector, with an ST Nucleo L053R8 board, with the aim of emulating an external disk drive. The board is a 3.3v but the Sharp PC is a 5v CMOS device, so I thought about introducing a bi-directional level converter in between, using one of this type: https://www.sparkfun.com/products/12009 (actually, one of the many clones).

image

Infact, the Nucleo board is 5v-input tolerant, but looks like its 3.3v output isn’t enough to drive the 5v input on the Sharp-PC.

After initially struggling to get the Sharp-PC properly communicate with the board, I found out that the level converter kept a high value on inputs, in spite of pull-down resistors, or pulling input down by software, e.g.:

    // input ports
    DigitalIn   in_BUSY   (D4);    
    InterruptIn irq_BUSY  (D4);    
    DigitalIn   in_D_OUT  (D5);    
    InterruptIn irq_DOUT  (D5);
    DigitalIn   in_X_OUT  (D7);     
    InterruptIn irq_X_OUT (D7);
    DigitalOut        out_ACK   (D9);
...
    in_BUSY.mode(PullDown);
    in_D_OUT.mode(PullDown);
    in_X_OUT.mode(PullDown);

So, I left the level converted only on the ACK line, which is an output from board to Sharp-PC, removing it from the BUSY, DOUT and XOUT input lines, as per this schematics:

sharp-ce140f-emul_v2
sharp-ce140f-emul_v2778×440 6.48 KB

My issue now is that this configuration is fine for the initial handshake only, which is working, but for exchanging actual data, lines are expected to be bi-directional, so the level converter would be required on data lines too.

Any alternatives? Should I use different board pins for inputs and outputs and “merge” them externally? I hope there’s a simpler approach!

Btw - my code is at GitHub - ffxx68/Sharp_ce140f_emul