MultiDrop RS232 possible with the Shutdown pin?

Eric,

The problem is that all 12 listeners will each have a 5k ohm input resistance. The total load on the master driver will be 5k/12 = 417 ohms. What power supplies available in your master system that I can use to power the solution?

The impedance of each slave driver in 'off state' is high impedance so the return data line to master is no problem.

How long is the cable and what is your desired data rate?

I can easily limit the number of listeners to 8 so then the input resistance is 625 ohms. That lowers the load on the master.
The cable length can also be somewhat limited but 10' between each node is probably plenty so 80' total.
I'd like the baud rate to be 115.2K but even 9600 is acceptable.

I have +5V for power at ~2A. Do the charge pumps generate enough current to pull against 625 ohms?

How is each slave driver put into the "off state" AND still be listening on the input?

After more datasheet review it looks like the MAX3221W will do what I want.
Tie EN_ pin low so the receiver is always enabled.
Then set FORCEOFF_ low except when the "chosen" node needs to drive the output.
Probably tie FORCEON to FORCEOFF_ to disable auto-power down which I don't need.
Thus, when the chosen node needs to talk back to the master it drives a GPIO high (FORCEON=1 and FORCEOFF_=1) to enable the TX driver.
Otherwise the pin is low which tristates the driver.

Is my understanding correct?

Is the 5K input resistor part of the RS232 spec?
If that wasn't there (or could be disabled) then I could have more drops.

Unfortunately my application already has a lot of Master units in the field so I can't switch to RS485. I am stuck with RS232 so I'm trying to be creative to get this very simple application to work with the existing hardware. I can always roll out more firmware to control the hardware but I need to find a solution that fits the existing hardware.

Eric,

The masters are already in the field but multiple slave compatible devices are not produced yet. Correct?

For this I suggest, using a RS232 device that has a receiver load control pin. The 5k load can be turned off. This will greatly increase the number of slave nodes possible. TI does not have this as an option at this time. Try Maxim.

In summary, for slave keep 5k off, keep receivers on at all times. Turn on driver only when instructed by master.

Correct - the masters are already in the field and we have a single board that has been connected in the past as a point-to-point.
But we're about to start a new design and having the ability to parallel a few boards is all we need.
8 would be plenty and the typical application will have 3.
I don't need lots of slaves and there is always 1 master.

The maxim chips are all pretty expensive and this is a cost sensitive application.

Will the MAX3221E (not W which is a typo above) receiver remain on when FORCEOFF_=low?
Looks like it is controlled only by EN_ - is that correct?