TTL Serial Bus using RS232 signaling - APPMOD? Or... Where is serial appmod pr

pwillard

I feel like I might be sweating over nothing...· but since I am only in design stage (I have one proto·unit built and talking to the PC) I still have questions before I get too far into this...

I've seen this topic about·SERIAL APPMOD PROTOCOL·come up a few times and even 1 or 2 references to the APPMOD serial protocol being used in some Nuts & Volts articles.·· Usually it's Jon Williams talking about·how it can be used for mulltiple listeners·but Ive never seen it explained·more than that.

In my design, I'm ·using it with·a PC COM PORT and a few SX28's attached, with the SX'28's·receiving commands from my PC using TTL level RS232 (possibly just being output from a MAX232).·· Each SX28 has a dedicated function in a display cluster.

What I would really like to do is have ALL of the SX devices listen on the same RCV pin "bus" without resorting to using a more complex bus like RS485 since all my separate functions will be within 6 inches of each other.

For example, using the APPMOD approach (which I really like, btw) 1 unit might respond to "!OG1" to display a "1" on a 16-Segment LED display.·Other units on the bus don't respond to !OG, so they ignore it.·

My concern is, how to I add multiple units to a single pin RCV bus and not have each·listener affect·the other from a loading/pullup perspective?· I'd noticed·that with the·SX APPMOD solution, there was a PULLUP on the serial·I/O·pin, but if each device attached also uses it's own pullup to 5V, I won't end up with the ideal design, no?·· I understand that the pin is either pulled low or tri-stated, but the external pullup is my concern.

(Yes, I know I could just experiement to find the answer, but what I was really searching for is a reference multi-drop serial APPMOD solution I could look at before I start more breadboarding.)

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There's nothing like a new idea and a warm soldering iron.

kjennejohn

Hi. How many slaves are connected? You can measure the voltage drop of the RCV inputs in the system with one slave, then the maximum number of slaves attached. If the drop is not too much (<5%?), you're good to go. BTW, if you have to use a pullup, just install it on the last slave in the chain. Put a two pin header in your design that disconnects the pullup from the circuit. Install the jumper clip on the last slave in the chain.

The other concern is distance. If I remember correctly, RS232 is good for 25 feet without a repeater. This is where RS485 excels. Using a LT485 or LT1481 you can do two to three thousand feet. Also, these chips will handle 32 loads, so no problem there! The address method is your choice, unless you're forced to use a particular protocol. Just start the host's stream with a unique address and let the slave figure out if it's his.

Later!
kenjj

Phil Pilgrim (PhiPi)

Your situtation is not unique. Those putting multiple devices on an I2C bus have the same considerations. Just use a single pullup. Typically, it would be put on the device farthest "down the line". But, since everything is so close, and line speeds are low, just stick it on the MAX232, and you should be fine.

-Phil

pwillard

Thanks! I appreciate the responses. I originally looked into RS485 but the overhead was huge when I started looking at common protocol implementations. I'm sticking with the KISS principle at the moment. I figure TTL levels will suffice for now.

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There's nothing like a new idea and a warm soldering iron.