Thanks heaps floobydust!

I have been considering going with an NXP UART as the 65C51 bug is quite annoying, and also disappointing that it (most likely) won't ever get fixed.
I've changed my mind a couple of times on which way too go. I'll go read up further and make a decisions soon.

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Greetings Professor Falken.

The TxD bug was discovered nearly eight years ago and WDC acknowledged its existence shortly after its discovery. Given that the 65C51 represents the state of the art of 1976 and there's little incentive for current designs to use such technology when so many better UARTs are available, you can be safe in concluding it will not get fixed. The cost to redo the mask would not likely be recovered in subsequent sales. WDC is probably sitting on a pile of unsold inventory that will eventually be scrapped and written off.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Realism... how sobering.
Fair call on WDC regarding the cost to fix the 65C51s and recovery given the current state of UARTs. Thanks for the info, it makes total sense.

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Greetings Professor Falken.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Now that you mention SPI, I have been considering it in the last couple of days. Daryl has an SPI interface I've been looking to build. Thanks! Looking into it...

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Greetings Professor Falken.

Thanks for the advice! I've added a 100uF cap near the MAX238.
I will look into the MLCC's as well.

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Cat; the other white meat.

Maxim and others make a lot of money out of the RS232 lie. The truth is that actual RS232 receivers (even the venerable MC1489) only need around 1.5V to register a signal and anything below 1.2V is not a signal, so there is no need for +/-12V. But wait! There's more! The MAX232 and other drivers don't even output that high, they are typically more like a noisy +/-6V, and yet even there we know that any RS232 device only needs TTL or even 3.3V logic levels to work.

I have designed many kinds of datacomms equipment over many decades and many specify "RS232" and so I dutifully use these 232 chips knowing that they aren't really needed at all. In fact I have used resistors from RS232 inputs back to the micro to limit any RS232 voltage current and this works better than a 232 chip. But as I mentioned, because much of this is professional equipment I need to be "compliant" to the standards, but my own and hobbyist stuff only needs to be "compatible".

BTW, RS232 chips are slew rate limited and have a 300 ohm resistance which back in the 1960's was meant to protect delicate and expensive transistors from shorts. Chips are very resilient to shorts yet they still have that 300 ohm resistance.

By way of contrast we are used to using logic levels over much greater distances at much greater speeds, so there is no advantage to using RS232, in fact there are lots and lots of disadvantages. Go MAXIM! Make more money!

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

These are simply facts vs "play-it-safe" tradition.

Look at the transistor input stage and RTM more closely and you will see that there is no magic there at all. Some early RS232 receiver chips did indeed have a 3V negative threshold and a 3V positive threshold and could therefore truly be said to be true RS232 and more immune to noise, but there were too many problems with this not working with that whereas the simpler 1489s worked with everything. The first charge pump type RS232 chips also used the same receiver thresholds as the 1489. Looking at a typical MAX232 datasheet shows that the Vit+ is 1.7V (or 1.5V) and Vit- is 1.2V and even the max and mins are well within compatibility with 3.3V logic levels. Heck I could run a 100 meter cable to these so called RS232 chips with just 3.3V logic signals and they would work just fine at 921600bd.

TTL is not low impedance and make terrible line drivers. Look at the NPN emitter follower that has a series diode to source the output. Not only was the voltage reduced from 5V down to around 3.6V or so, but also they could not source much current, although the NPN for the sink stage could handle more current. This also suited the input stage too but there was always a big problem with fan-out where an output had a limit as to how many inputs it could drive. That's why it was common to have pullups and active low signals rather than the more conventional pull-down active high.

A resistor to limit the current is all you need for the input plus a pull-down on the RS232 side. Driving RS232 COM ports on PCs from logic, even 3.3V logic actually performs much better and at much higher baud rates than slew-rate and current limited RS232 drivers can handle. I just updated the firmware on one of my older designs that use single-mode fibre to link RS232 ports over long distances and those RS232 signals look slow, noisy, and terrible slewing from -6 to +6V volts. All high speed stuff these days uses much lower voltages and thresholds and differential drive (LVDS etc). RS232 drivers top out at a measly 120kbd and some maybe higher, but not too much higher due to the inherent limitations of a 60's standard that considered 1200bd really hammering.

I said all this simply because the "interweb" is full of "gotta have this MAX type chip" for RS232 but the truth is you don't.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Oh boy .... vendetta? You really don't know if you say that EIA-232 is "low-impedance" when all the datasheets say "300 ohm" output resistance. That is never "low impedance".

The original 232 standard offered noise immunity, albeit at very low speeds (20kbps max although 250kps is still slow) over relatively short (50 feet max) unbalanced lines but that is the point. Even if properly adhered to with a +/-3V receiver sensitivity the standard is sadly lacking but worse still, receivers do not have anywhere near this level of "noise immunity" with only a few hundred millivolts of a noise band centred around +1.5V or so. That's because it is a simple transistor input stage with very limited hysteresis.

Drive that long cable instead with 0-5V into a CMOS receiver with a termination resistance and you will find that it can run at higher speeds etc. One of my early designs were R6511AQ based POS terminals and before I started using RS-485 for multidrop, I used a simple PNP driving a multidrop line from +5V for hundreds of meters over standard 4-core data cable. They just worked although RS-485 allowed much higher speeds and more immunity to noise.

I put MAX chips in decades of commercial product so obviously I do what I need to do, and don't do what I don't need to do. Do the science, do the math, connect the chips up to the scope and meters etc and vary those input voltages which will confirm exactly what I have said.

I've said what I've said for hobbyists overly concerned with meeting the RS-232 standard by using any brand of special RS-232 chips when the simple boring truth is you don't need to.

Fortunately none of us "has to" use RS232 in hobby designs even with the few old bits of electronics that still use them as I have clearly outlined. Irked though I may be, I have to use RS232 for product compliance in the air-traffic and transport industry which still uses a lot of this stuff, even in major international airports. Using RS232 there doesn't cost me a cent, instead it makes me money.