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Original question:

SN74HCS244-Q1: Jtag demultiplexer for 1~3m cable

SN74HCS244-Q1: drive 1m twisted pair

Lijia Zhu
Expert 3570 points
Part Number: SN74HCS244-Q1
Other Parts Discussed in Thread: SN65LVDT41, SN65LVDT14

Hi team,

My customer wants to drive the SPI signal between two separate boards. There are four twisted pairs between these two boards and the length is 1m. The twisted pair is from an Ethernet cable. 

Is it possible? Do you have some suggestions on this?

B&R

Lijia

  • 0
    TI__Guru* 96591 points

    Hi Lijia,

    Yes, I can make some suggestions.

    I would recommend to use a source termination like this:

    Rs is used to match the source (HCS244) to the transmission line (your twisted pair). Most twisted pair wires have impedance around 100 ohms, and the HCS device has an output impedance around 30 ohms, so I'd start with Rs = 75 ohms and see how that works. If they know the characteristic impedance of the twisted pair (transmission line), then they could match it more precisely based on the datasheet provided output impedances for the HCS device.

  • 0
    Guru 243220 points

    If you already have pairs, then it makes sense to use differential transceivers, e.g., SN65LVDT41/SN65LVDT14. This is very helpful if the environment can be noisy.