Is the world ready for TTL RS-232 serial?

[hansknec]

You're right of course. But I'm really reluctant to send out a RS-485 system based on my own experience with the termination problems. Its also harder on the end user because they will need a 232 to 485 converter for each network. I should probably do some reading, but I would think I could get an RS-232 chip and tri-state the drive to high Z, and thereby effectively allow multiple units to sit in parallel. The down side is that I haven't found any 232 recievers with a high input Z. They all seem to have 5K by design.

John

[NavMicroSystems]

... I'm really reluctant to send out a RS-485 system based on my own experience with the termination problems. Its also harder on the end user because they will need a 232 to 485 converter for each network

There is no big difference:
You'll either need an RS232 Level Shifter or RS485
Transceiver.


RS232 is actually not designed for your kind of application.

However,
a number of Listeners on a the same bus with a single Talker shouldn't be much of a problem.
(Even boosting the fanout of a standard driver is no real problem)
But you should bear in mind that you are most likely running off specs
and the price you have to pay is less compatibility and reliability.

With more than one Talker it is going to be difficult.

(There is no standard RS232 Driver with Open Drain).

Imagine the following scenario:

You presume: any device your product could be linked to is within the specs,
so there shouldn't be much of a problem if your product is slightly off specs.

Now the Designer at the other end has had the same thought
and is as well slightly off specs (but the opposite way)

Guess what:
Those two devices will (most likely) not be able to talk to each other!
(Worst case they could even destroy each other!)

So again:
there is a reason for standards being exactly defined.
And a good design is supposed to be within the specs.

[hansknec]

With more than one Talker it is going to be difficult.

(There is no standard RS232 Driver with Open Drain).

But there are many RS232 tranceiver chips that have controllable shutdown modes that put the transmitter in a high Z mode and still pass the receive characters. (See MAX2211E) My PICs only respond when specifically addressed and asked a particular question. My code would only enable the transmitter when it has determined that it's time to talk. The master station (PC) polls all slaves (PICS) in sequence and only skips if a reply is not sensed within a specified time period.

I understand your wanting me to stay within the specs of the standard, but in my mind I'm secretly hoping that you and others would agree that such a system could indeed operate reliably using RS232. I am really not very fond of RS-485. I avoid it like the plague when purchasing instrumentation from others, and I would expect that many of the potential customers would feel the same.

I have purchased some RS-232 devices from ZABER.com (linear actuators) that use standard RS-232 to one or many devices in series. They had an interesting approach to their networking of the devices where only the first device would receive the true RS-232 signal and then it would retransmit the signal to the next one down the line. In this approach all devices had to actively pass the signal regardless of whether or not they were the intended target.
I guess there are about a bazillion ways of doing it.

[NavMicroSystems]

...In this approach all devices had to actively pass the signal regardless of whether or not they were the intended target.
I guess there are about a bazillion ways of doing it.

I wouldn't like this solution, because if one device on the bus fails all others further down the line would be disconnected.

Sure, there are many ways...

I don't know why you dislike RS485 that much, it is proven to be reliable.

And of course you can setup a bus using a kind of RS232,
but if it is off specs don't call it RS232,
because RS232 implies it is within the RS232 specs.
(And people may tend to hook it up to standard RS232 devices, which may cause problems)

Imagine you connect a standard driver (that drives the line to +/- 12V in idle state) to "your" kind of bus . . .

If you call it John'sSerialBus
noone would hook it up to a standard serial port without having had a look at the Manual first (hopefully).

[hansknec]

I wouldn't like this solution, because if one device on the bus fails all others further down the line would be disconnected.

I agree.

Imagine you connect a standard driver (that drives the line to +/- 12V in idle state) to "your" kind of bus . . .

I Agree.
I thought it would be clearly apparent that the user could only connect my devices because the 9-pin serial gets converted to RJ-11 and would only be able to plug into my network. Each device on my network has two RJ-11 sockets that allows the units to be daisy chained on to the network (in parallel).

My existing devices are RS-485 now and they work, but I was just trying to come up with a simple to integrate RS-232 solution to save the end user some $$.
-John

[hansknec]

http://pdfserv.maxim-ic.com/en/an/AN723.pdf

A detailed tutorial about this very discussion. I wish I had googled "multidrop RS-232" before posing the question, but instead I had googled "multiple RS-232 devices on a single network", which got me nowhere.

So the end answer is that there are specific RS-232 Tranceivers designed just for the very purpose of developing a multi-drop RS-232 system (that remains within the EIA-232 standard)

The MAX3322E is one chip designed for this purpose. ($3 each)

Thanks for your input Bruce. I will now have to weigh all my options and perhaps order a dozen of these chips to build a prototype network.

-John