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SN74AVCH4T245: Bus hold

Part Number: SN74AVCH4T245
Other Parts Discussed in Thread: SN74AVC4T245


I am seeing some weird issues with SN74AVCH4T245PWR. I am using this device as shown in the illustration below where the right side is a common shared line on a backplane with 16 cards. When I have one card it works great but when I insert another and don't drive the lines at the same time I see the tristated state getting squished from a 3.3V pull up (Not shown in figure below) to 1V. I think this maybe related to the bus hold feature. Will this work as a replacement? SN74AVC4T245PWR

Signal CONT1_CARDSTATUS_ENA enables the bus when commanded by a common controller and otherwise I expect the 3 lines to be tristated. A pull up at the other end understands it as logic 1 when not commanded. When we want to drive a status we drive a 0.

  • Hello,

    Bus-hold circuits are intended to replace pull-up or pull-down resistors and usually shouldn't be used together with them.

    The SN74AVC4T245 will be a direct replacement that eliminates the bus-hold circuitry -- you will just need to have some kind of termination on all of the bus lines (pull-up or pull-down) to ensure valid states during the hi-Z operation. It sounds like you already have that setup, so it should be just a quick drop-in replacement.

  • Thanks a lot,