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CD40106B: Using RS232 transceiver or CD40106B logic ?

Part Number: CD40106B
Other Parts Discussed in Thread: TRS3243, GD75232, CD4010B

Hello,

I have a motherboard that may be connected to an external peripheral, using several signals to communicate between them (UART + controls). The peripheral may draw some power (up to 2A), cable used is a classic ribbon cable (1.27mm pitch), with max distance about 1 m (4 ft), max baudrate 115200bps.

I initially choose a RS232 transceiver to communicate between them (TRS3243 / TRS3238), but I'm looking to replace this "costly" solution by standard logic. I have on both board a power supply rail 10-15V, so the CD40106B may be used. With schmitt trigger inputs and such high VCC voltage, I think I may have a good noise immunity, and also immunity to ground shifts that may occur when peripheral draw high current. Moreover, the EMC emissions may be reduced since there is no charge-pump.

I will add serial resistor on the output of the drivers, and also on the inputs. I have to add extra ESD protection, since CD40106B logics limited to 2KV.
Can I simply add a TVS array that may clamp voltages to 30V during surges ? When a ESD surge comes in, it may be clamped to 30V, but does the 2KV ESD protection enough to absorb it, or have I to add additoinnal current limiting serial resistor and capacitance ?

Do I miss something ?

Thank you
Aurelien

  • The best solution for noise immunity are differential signals, i.e., LVDS or RS-485. This requires more signal lines in the ribbon cable. (RS-485 transceivers have strong ESD protection.)

    Using a high voltage is not as good a solution because all signals must share the same ground. The CD4010B will work (the CD40106B is not a level shifter), but when you have positive and negative high-voltage supplies, plain RS-232 transceivers without charge pump like the GD75232 or SN74185 have more ESD protection.

    If you have external ESD protection, the series resistor should be placed between the external diode and the device so that the current of the remaining 30 V spike is harmless.

    For low-frequency signals, a capacitor is the cheapest way to filter out high-frequency noise (including ESD).