Serial (RX & TX) pins--pullup or active high?

I have an application where multiple Arduinos communicate with a central Arduino via serial. It seems that the central TX pine can be connected to multiple remote RX pins (I haven't figured out what the max fan-out is).

However, the opposite doesn't seem to work, eg, multiple remote TX pins tied to the single central RX pin.

This tells me the RX pin is a passive pull-up, while the TX pin is an active high. Is this right?

Do I need to buffer all the remote TX through a multiple input AND gate?

I was under the impression that the TX pin was an open collector output with a passive pullup, so that it was only active low.

Both directions are active drivers. High level is the inactive state, low level is the active state. Tx's on the slave devices that want to communicate with the master's Rx pin can connect to the cathode of a diode, with all the anodes connected together to the master's Rx. When any one goes low, it pulls the anode low. Master will probably need 2.2K to 4.7K pullup resistor to bring the line high reliably/quickly. Or buffer each one with one gate of a 74AC125 (24mA drive capability), enable the output when the slave wants to drive the line. Tx on the master can drive a lot of current - what kills the signal eventually capacitance and inductance from all the wiring connecting stuff up. If you have a lot of devices, then adding RS485 driver and connecting everything up via twisted pair will improve signal integrity a lot so high speed comms can be maintained.

Geeze, why didn't I think of diodes? Duh. Thanks.

As far as RS485, I'm trying to hold to 4 wires (TX, RX, Gnd, Vcc), so I can't do differential. I have Ethernet wire to each location, but I'd like to reserve those cables. If I find that I've got too much wire, I'll try the old TTL buss drivers which, I recall, had four amps in each package.

There are 9 remote Arduinos. I could step up to a Mega and put two on each serial port, plus a third only 4 feet away on one of them. (this is a radiant heat system with 18 zones)

I just found a handful of 1N649 switching diodes in the junk box. Works like a charm.