In SX Microcontrollers, SX/B Compiler and SX-Key Tool, robotworkshop wrote:

I've got this partially working but there are still some bugs to work out.  An old Handyboard is the host in this case and it is using one of the optional libraries "pa7_9600.icb" to send out serial data to either a SpeakJet chip or an SX48 chip used as a slave processor.  It is using the method described above using a 4066 to switch which chip the Handyboard will be connected and send data to.  These are all logic level signals and i'm not using any drivers like the MAX232, etc on the line.  The first time through I forgot to add any pullup/pulldown resistors on the line so that the input to the device not currently selected would float (not a good thing).  Even so, I could send data to the SpeakJet ok and also send data to the SX48 chip.  As a test the SX48 would just display whatever byte was sent to it on a set of 8 LEDs.  Most of the time the SpeakJet and SX48 would perfectly receive what was sent.  However, I noticed that when the SX48 wasn't selected the pattern of LEDs could and would change sometimes on their own.  That goes back to the lack of any pullup/pulldown resistors.  So, I added pullup resistors on both the serial in on the SpeakJet and the SX48 chip.  The SpeakJet worked fine but the SX48 always received 255 (all bits on).  Just for the heck of it (it is a prototype after all) I made both resistors pulldowns.  In that case the SX48 would display the proper patterns on the LED but this caused another issue.  Switching between the two devices caused some random characters to appear on the LEDs and send odd codes to the SpeakJet.  I'm sure that was caused because the default state should have been high and when switching between the two devices the pulldown caused a transition which looked like the start of a new byte coming in.  This didn't happen when the pullups were used or there wasn't any on the line.

I'm going to have to go back to the pullup resistors but have to figure out why the SpeakJet seems to get the data ok yet the SX48 always receives 255.  Either that or look at a whole different method for sharing out a single serial (output only) port to multiple devices.  I'm going to review the whole thing again and double check all the wiring but perhaps someone will notice something I may have missed.

If you have any suggestions i'd appreciate it.

Robert

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