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I have an existing product with a spare RS232 port on the rear panel. This is connected in the standard way to a MAX3232.

I would like to use this to connect to another device that is TTL level, so I don't want the output to go below GND. The high level is OK, as it's clamped at the far end via a resistor and Schottky diode to Vcc.

The PCB designs can't be changed, and I would very much prefer to avoid making modifications by hand. The inversion isn't a problem; the software can handle that.

However, changing the BOM isn't a problem. I haven't tried yet, but is it possible to achieve this by not fitting C2, or something else (tying V- to ground with a 0 Ω resistor, etc.)?

Has anyone ever done this? It would be so helpful if it's possible.

I know it's not ideal, but it would be a huge help.

In an ideal world, I'd spin a new PCB. Sadly, I'm not in that world for this project.

ocrdu
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StephenC
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2 Answers2

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Unless you're bit-banging the RS-232 or unless your MCU has a inverted output function I doubt this will work (ah, it does, from your comment, I remember that a few have that function.. very handy if you need it).

But anyway, I think if you both leave out C2 and ground V- it should work.

If you're going that far, why not just omit the chip, and short the ins and outs with a blob of solder and a little 'U' shaped jumper respectively? ..and tack the jumper down with a bit of adhesive. I've see far worse hacks that were 100% reliable in the field.

Spehro Pefhany
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    Hi. Thank you for that. The issue is that the expertise available at the far end doesn't stretch to that level of mod. I can issue a BOM change without issue, but there is nobody there who has the skills to effect the hand soldering process. I know it seems ridiculous, but this is the problem I've been tasked with solving at the BOM level, if at all possible. – StephenC Nov 14 '22 at 14:22
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Not sure of the circuitry around your MAX3232, but perhaps you have existing position (possibly on the receiving device, possibly in the cable/connector) where you can add a diode, per the following application note.

TI's Application Note 972 Inter-Operation of Interface Standards says:

12. TTL to RS-232
TTL output levels can directly inter-operate with certain RS-232 receivers. This is true since most RS-232 receivers support a tighter threshold specification than required by the RS-232 standard. The RS-232 standard specifies that the thresholds are between +3V and −3V, however, most thresholds are centered around +1.5V. If this is the case, then standard TTL levels (High > 2.0V and Low < 0.8V) will be detected correctly. One note of caution is that the TTL gate will be loaded with the 5 kΩ load instead of a standard TTL input load. The TTL gate driving the RS-232 receiver must have adequate drive capability to obtain the correct levels with the RS-232 receiver load. This connection is illustrated in Figure 8.

13. RS-232 to TTL
RS-232 output levels are polar, and therefore they swing around ground. This negative swing typically prevents direct inter-operation to TTL inputs which prefer positive voltages only. To clamp off the negative swing a diode may be used to clip the negative swing but will load down the driver when the diode is forward biased. This is typically acceptable if the driver employed provides a relatively tight current limit in the range of 10 mA. Figure 9 illustrates this inter-operation with a diode clamp.

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jonathanjo
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