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MAX3221: Meter_TX Signal not attaining +5.5V & -5.5V levels

Srikrishna Y H
Prodigy 30 points
Part Number: MAX3221

When IC is interfaced with L&T Energy Meter, Meter_TX Signal is attaining 3.48V & -4.2V rather than reaching +5.5V & -5.5V levels. I'm attaching Scope Output for your reference as well as Energy Meter - MAX 3221 Schematic for your reference. Please provide clarification for this issue at the earliest. Awaiting for your reply.

Meter Side Picture for your Reference

1. Tx_Meter is connected to RIN of MAX3221

2. DTR is Connected to +V of MAX3221

3. Gnd is Connected to -V of MAX3221

Scope Output for Reference

Yellow Signal - METER_TX - 3.48V & -4.2V

Blue Signal - MCU_RX

  • 0
    TI__Guru 52242 points

    Hi Shrikrishna,

    Yeah - from the description the schematic is done incorrectly.

    1. RIN should not have external pull-down on same bus. RIN is rated from 3k to 7k input impedance - minimum input impedance is 3k for RS-232. Adding a 2K resistor in parallel will - best case - result in a ~1.5k load - which is half the minimum input impedance required by standard - you will not get the full voltage swing. 

    2. V+ is not an input pin - its the charge pump output - which should only be connected to a capacitor to ground - this is the same for V-. They should not be connected to anything else or you risk improper setup of charge pump which could lead to application failure and potentially part damage. 

    3. As stated above V- is a charge pump output and should not be connected to ground - it should be connected to a capacitor connected to ground.

    Proper Schematic setup is shown below:

    If those problems are corrected - I'd imagine you probably won't see the issue you are seeing. 

    So in conclusion: no external pull-down on RIN should be used, V+ and V- are only to be connected to capacitor to ground as shown in figure above (values of which are shown in table at bottom of figure)

    Please let me know if this fixes your issue or if you have any additional questions.

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 30 points

    Hi Parker,

    First of all , Thank You for reverting back.

    Went through your reply, hence wanted to clarify few things.

    1. We haven't connected any Pull Down Resistor(2K) on RIN, its part of Meter (Present Inside Meter as a part of circuit).

    2. We have connected Capacitor of 0.1uF on V+ & V- and Grounded them as per datasheet and above attached schematic by you, but additional thing which we have done is used V+ & V- Output as 12V and GND for Meter (DTR & GND Signal in above meter circuit). Reason why we have done so is because Meter gives Output which toggles between 12V & 0V state and MAX3221 RIN ideally requires signal in RS-232 form, one toggling between +5V to -5V. Hence we have done so. 

    Please Do Clarify.

  • 0 in reply to Srikrishna Y H
    TI__Guru 74252 points

    Srikrishna,

    Parker is currently out of office and the US has a holiday tomorrow (04/07/2023), so please excuse any delay. We will get back to you with a response next week.

    Regards,

    Eric Hackett

  • 0 in reply to Srikrishna Y H
    TI__Guru 52242 points

    Hi Srikrishna,

    1. Regardless of if you have added that resistor or not - if it is part of the bus - RS-232 is not going to be compliant - as that 2k puts the bus out of compliance. RS-232 parts most likely will not work in this application if that pull-down resistance is present. 

    2. These pins aren't really rated to provide power to other devices - they are specifically the boosted supply for the transceiver itself.

    So - if that 2k resistor is there no RS-232 part is going to be recommended because they aren't rated to drive that low of a load.

    Best,

    Parker Dodson