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Connecting 5V signals to P2? — Parallax Forums

Connecting 5V signals to P2?

marsman2020marsman2020 Posts: 71
edited 2018-11-21 05:24 in Propeller 2
What would be the simplest way to connect a 5V signal to the P2? Are there diodes in the pin circuit such that a simple series resistor to limit current in the diode would work? Or is a resistor divider or active level shifter required?

I have an existing P1 board with 74LS series logic running through series resistors to the P1. Looking to build a modified version with a ribbon cable connection for the Eval board.

Comments

  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2018-11-21 05:38
    Voh of any ttl is typically around 3.5V anyway unlike 5V CMOS which does output 5V. I would use a resistor but Chip reckons 5V is unlikely to be a problem. If speed is not a problem you can even use 100k which is what i do for 24V PLC type inputs.

    If it worked for the P1 it will work for the P2
  • cgraceycgracey Posts: 14,133
    edited 2018-11-21 05:54
    5V pull-up through a weak resistor is not a problem. Don't drive the Vxxxx pin at 5V, though. I think you would shorten the life of the chip.
  • jmgjmg Posts: 15,148
    marsman2020 wrote: »
    I have an existing P1 board with 74LS series logic running through series resistors to the P1.
    Can you still buy 74LS ?

    marsman2020 wrote: »
    Are there diodes in the pin circuit such that a simple series resistor to limit current in the diode would work? Or is a resistor divider or active level shifter required?

    What speed, and how many channels ?
    There are diodes there, so a series R can be used, but you need to watch the total pullup current does not exceed the minimum P2 drain, otherwise the 3.3V can start to lift.
    For fastest and lowest power, the dual-supply levels shifters work very well.

  • potatoheadpotatohead Posts: 10,254
    https://www.futurlec.com/IC74LS00Series.shtml

    Besides, whether one can buy them doesn't actually get at the problem, given they are right there on the board :D
  • marsman2020marsman2020 Posts: 71
    jmg wrote: »
    Can you still buy 74LS ?

    Yup, all day long from Digi-key brand new.
    What speed, and how many channels ?
    There are diodes there, so a series R can be used, but you need to watch the total pullup current does not exceed the minimum P2 drain, otherwise the 3.3V can start to lift.
    For fastest and lowest power, the dual-supply levels shifters work very well.

    14 pins, max speed is 10 MHz (limited by S-100 bus clock)

  • tonyp12tonyp12 Posts: 1,950
    Or you could use a dual-supply 595 serial-parallel
    https://www.mouser.com/ProductDetail/Nexperia/74LVC8T595PWJ?qs=sGAEpiMZZMtsbn1GaJysl%2bx2BnO01/vCPUZO8M/3/lM=
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