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270Re: [kminternals] Signal voltages for Brother 9xx

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  • Steve Conklin
    Jan 22, 2013
      Off the top of my head, here are my thoughts on this . . .

      If you look at the specified input voltage levels for 4xxxB series CMOS operating at 5V, typical values are something like
      "input LOW must be less than 1.5V and input high must be greater than 3.5V". This is in order to meet the specified switching speeds of the device. However, the serial port switching is pretty slow, relative to the max that the parts are specified for.

      The reality is that most of these CMOS devices switch at around VCC/2, which is 2.5V with a 5VDC supply.

      If the 3.3V FTDI puts out 2.8V typical and 2.2 minimum, this means that using this device would 'typically' work, but not in all cases and perhaps not reliably.

      If you operate systems near the margins like this, they may work most of the time, but the switching margins of both the source and the receiver change with temperature and supply, and you are operating with little or no noise margin. So problems may arise, depending on temperature, cable length, and other factors which are very hard to determine. These are difficult to tourbleshoot.

      This is why I recommended the 5V FTDI cable.

      Now - having said all that . . . I have never heard a report of the 3.3V FTDI cable NOT working for anyone.

      But just because it's always worked until now doesn't make it right.

      In my opinion, good engineering practice is to use the 5V part.


      On Sat, Jan 19, 2013 at 4:18 PM, Davi Post <post@...> wrote:

      One point I'm not too clear about is whether to use the 3.3 volt or 5 volt FTDI cable. We've been using the FTDI TTL-232R-3V3 cables as recommended in the Adafruit Electro-knit tutorial, and haven't noticed any problems.

      But Steve's Electroknit Serial Connections (Signal Voltages section) recommends 5V signals. Steve figured out that the serial port is driven by a CMOS device. 

      Unless this is a CMOS/TTL device, I believe it may need higher voltage signals than TTL.

      The FTDI TTL-232R Datasheet [PDF] shows the electrical parameters for the TTL-232R-3V3 on pages 11-12.

      The spec for Voh (Output Voltage High) is minimum 2.2v, typical 2.8v. Could this be too low for a CMOS input?

      I don't know which chip in the knitting machine is connected to the serial port inputs. If they go directly to the MB55881 microcomputer, its datasheet shows a minimum Input High Voltage of 0.7 Vcc, or 3.5v.

      Seems to me we should be using the TTL-232R-5V cable. I'm not an electrical engineer, so I hesitate to recommend a different cable from the one Adafruit specified. Can any of you shed some light on this?



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