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MAX3221: Using more than one MAX3221 together

Callum Bassie
Prodigy 20 points
Part Number: MAX3221
Other Parts Discussed in Thread: TRS3232E

I'm trying to get 2 devices to talk to one. The datasheet sates that "When MAX3221 is unpowered, it can be safely connected to an active remote RS232 device" but it doesn't seem to work when more than one device is connected. I have a main device which is requesting information from two devices. I'm toggling the FORCEOFF until the processor for each device receives the correct ID to enable communication to the main device. RS485 would be more suitable but we are trying to avoid twisted pairs. 

  • 0
    Guru 287370 points

    RS-232 uses point-to-point connections. A driver expects only the load of a single 5 kΩ receiver.

    RS-485. If you do not want to use pairs, use I²C or open-drain UARTs (which are much more sensitive to noise).

  • 0 in reply to Clemens Ladisch
    Prodigy 20 points

    In theory, if the load resistance was increased on each listening device then it should work (placing a 1k resistor in line with each RIN) to give a total load of 3k for the two devices.

    I can see the datasheet states 3k as the minimum input impedance.

    Would this cause problems when only one device is used?

    The main device already contains the TRS3232EIDBR and can not be changed.

    RS-485 is impractical for our end product and open-drain UARTs failed BCI testing. 

  • 0 in reply to Callum Bassie
    TI__Mastermind 47357 points

    Hi Callum,

    So you are assuming that the receivers will be 5k - that's not guaranteed - they could be as low as 3k - which means you would need to put 3k resistors in series to ensure that the minimum impedance seen by bus is 3k - that means worst case you cut your received voltage by half which is going to add a lot of risk in terms of proper communication. 

    According to RS-232 standard valid voltage levels at receiver are -3V to -15V or 3V to 15V - the MAX3221 has slightly more favorable thresholds as below 0.6V (worst case 3.3V supply) that is counted in the -3V to -15V category and if the signal is >= 2.4V than the transceiver will consider it in the 3V to 15V category. The thresholds are the same on the TRS3232E as well.  However here is the main issue - the driver is typically outputting +/-5.4V volts with these devices - that means that worst case scenario you could see 2.7V without considering loss from cable (and worst case 2.5V without considering loss of cable) - so any cable added could potentially add risk of communication failure - transmitting a logic high (which is negative on the bus) shouldn't be as big of a concern since the VIT- threshold on the receiver is positive - so -2.7V to -2.5V isn't great. The other thing to note is that 2.7V technically isn't RS-232 compliant - it may work with the receiver - but it does violate standard. 

    Basically RS-232 is point to point and you won't get good results trying to have multiple devices on one bus for RS-232. That is when we suggest moving to RS-485 and typically since RS-232 is slow and short distance the RS-485 solution can neglect certain best practices relating to emissions/EMI/EMC - that depends on the exact needs but the reason I am really pushing this is because there is no good solution to make RS-232 multi-drop whereas RS-485 or RS-422 both are designed to handle multi-drop configurations. 

    Please let me know if you have any other questions!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 20 points

    Hi Parker,

    Is there a known part or possible substitute that doesn't contain a 5k load resistance on the RX?

  • 0 in reply to Callum Bassie
    Guru 287370 points

    There are no integrated RS-232 transceivers with high-impedance inputs.

    If you have proper supplies (±5 V or larger), then it would be possible to build drivers and receivers with comparators.

  • 0 in reply to Callum Bassie
    TI__Mastermind 47357 points

    Hi Callum,

    Unfortunately not - all the RS-232 receivers should include the 5K ohm pull-down on the receiver pins - the vast majority of datasheets are 3k min, 5k nominal, and 7k max - but in this case you have to look at the minimum for guarantee which will always be spec'd to 3k due to standard. TI at the very least doesn't have any higher impedance RS-232 and throughout industry I haven't seen one with a guaranteed higher level. Essentially RS-232 just doesn't really work effectively with non-point to point applications. 

    Best,

    Parker Dodson