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Quadruple 74HC257 2-to-1 multiplexer with common select line, making 16 switches in one chip. Used in dual slot MVS systems to select one of the two C ROM (and S ROM ?) buses.

Select signal comes from one of the "SLOT" outputs of NEO-F0 ?


Information provided by Channelmaniac.

File:Neo-257 pinout.png

OpenOffice Draw file: File:Neo-257.odg

  • A*,B*: Inputs
  • Y*: Outputs
  • Select: Output A if LOW, B if HIGH.
  • Some pins marked as ground could be "enable" pins
    • 35 is active high enable atleast for Y8~Y15
    • 33 is active low enable atleast for Y8~Y15 (confirmed from NEO-257 connection on a 2 slot). In the example from the pic it appears to be hardwired to always be enabled.