i'm currently using this reference schematic to wire up 10 RS232 slaves in a "multidrop" configuration (The slaves are some ATMEGA328P with MAX232 Trancievers in order to make a real RS232 network).

Multidrop net

I have some doubts regarding the diode to put on TX line of slaves: i've tried using LL4148 (Datasheet here) but nothing works, the slaves will not work as expected.

So, i've taken a BZX85C5V1 Zener diode and i've replaced the LL4148, now the slave works as expected: receiving and sending messages to the master.

What i'm missing? maybe i've chosen a wrong diode (LL4148)? which diode i can use in this multidrop serial network configuration? Why 5V a zener (BZX85C5V1) works?

  • 1
    \$\begingroup\$ RS232 is not a multidrop bus so this is not the intended use case for it. At what speed you are trying to communicate? How long is the bus, if there are 11 devices in total? What is the master/host of the bus, does it have MAX232 as well or something else? Are the diodes the correct way around? \$\endgroup\$
    – Justme
    Commented Jun 16, 2020 at 18:52
  • \$\begingroup\$ Yes, there are 11 devices in total, each slave has an atmega 328p with respective diode on tx line. The master is an ESP32 with relative Max 3232 without diode on TX and RX. As master, it's only swapped the Rx and TX swapped to the slaves \$\endgroup\$
    – VirtApp
    Commented Jun 16, 2020 at 20:04
  • \$\begingroup\$ The outputs of the Max232 are bipolar ( +/- 12v ) so a diode 4148 does not work unless you add a pulldown resistor to -12v (you can take it from one of the max232). The zener is not the best solution. \$\endgroup\$ Commented Jun 16, 2020 at 20:10
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    \$\begingroup\$ @VirtApp the resistor must be at the master RX. A pulldown is needed because at idle (all slave outputs to -V all diodes are open circuit and the Master RX will be connected to ground through the RX resistor typ 5k. The zener solution is bad because from one side you have an asymetric bipolar signal and from the other side you will have an excessive current curculating between Tx outputs \$\endgroup\$ Commented Jun 16, 2020 at 21:06
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    \$\begingroup\$ 10 meters of what kind of cable? Twisted pair, something else? How much capacitance the cable has? @PaulGhobril The MAX3232 receiver input has schmitt trigger so it has hysteresis and 0.6V is only the the minimum for low-going threshold. Typically the threshold is 1.2V with 0.3V hysteresis. If the asymmetric drive with cable capacitance is the issue, it can help to discharge the capacitance though. But in reality, RS232 tranceivers are really meant for one-to-one connection, a driver is not meant for multidrop, so the driver may have trouble driving 10 receivers too. \$\endgroup\$
    – Justme
    Commented Jun 16, 2020 at 21:16

1 Answer 1


The diode is on pin 13 and that is the wrong pin as that is the RS232 input pin R1IN on MAX232. Diode should be on pin 14, which is the RS232 output pin T1OUT.


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