Dear TI Team,
We are using 2 - MAX3221 (Rev M) devices in our application that requires communication with an embedded module (RS-232 port) and also a PC (RS232). The baud rate is 115.2kbps. The system is powered with a 3.3V supply.
In our application, we use a microcontroller with a single H/W UART module to drive both these MAX3221 devices. In order to facilitate this, we intend to use the following configuration.
- Tie the FORCEON pin of both the MAX3221 devices to 3.3V
- We toggle the !EN and !FORCEOFF pins to be able to receive/transmit from PC or other embedded module. We have the following commonly used scenarios,
Case (A) - Receive from Embedded Module and Transmit to PC
Embedded Module side - MAX3221 configuration
- FORCEON = High
- !EN = Low
- !FORCEOFF = Low
PC side - MAX3221 configuration
- !EN = High
- !FORCEOFF = High
Case (B) - Receive from PC and Transmit to Embedded Module
We will be switching b/w the configurations depending on when we receive the interrupt from either the embedded module or PC.
In context of the above, i had the following queries
1) The datasheet specification is 100us for driver enable to valid data at the output and 200ns for receiver enable to valid output. Assuming we follow these specifications, do you see any other limitations with our proposed use scenario?
2) The datasheet specs the typical values, is there data on the max value? We intend to assess the max value and design the system accordingly.
3) On a separate note, do you see any limitation in using MAX3221 to drive about 20ft of cable @ 115.2kbps (this is the max distance b/w the embedded system and our controller)
Because there is no maximum value, I suggest a conservative 400uS to allow the charge pump to turn on and stabilize.For the receiver the same guard band can be used which is 800nS.
Provided that the cable capacitince is less than 1000pF, the signal will be good.
Regards,Ronald MichallickLinear Applications
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For question #1.
The multiplexing scheme will work well provided that the remote RS232 ports do not transmit while your application is looking at the other RS232 port.
Thank you, appreciate your prompt support. These numbers are ~4x higher (400us for TX drivers and 800ns for RX) and are probably NOT characterized. Please advice. Our budget looks a little tight if we were to use these numbers and hence wondering if some sort of Silicon/Characterization data is available for the same?
Unfortunately after ordering MAX3221 parts, we saw the following on the TI website " TI recommends the MAX3221E in new designs. The MAX3221E is an improved direct drop-in replacement for the MAX3221". Do you see any serious limitations in pursuing with MAX3221? What are the enhancements in MAX3221E (as against MAX3221) vis-a-vis the application use scenario that i had described earlier?
With Best Regards,
The test program does not check power up time so I can't use data logs to get a timing distribution.Because I had no data available, I gave very conservative guidance.
The start up time is likely faster when VCC is 5V.What is your VCC and what are your timing requirements? I may be able to lab test some samples.
The "E" suffix gives improved ESD and EOS protection on the RS232 port pins.
The system VCC is 3.3V (Typical). For the transmit side, we should be fine with 200us budget. Appreciate if you could help check the same in the lab and let us know.
I checked several samples of MAX331 devices.The force off release to DOUT active time delay of the samples (14 units) varied from 69uA to 72uS (VCC=3.3V).
I attached waveform from one unit for DOUT, V+ and V-.
The output load does not change the time. However, variance in values for charge pumps capacitors can affect delay.Lowering VCC to 3.0V increase delay by 16uS; Increasing VCC to 3.6V lowered delay by 13uS. So VCC is a factor.
I expect 200uS to be sufficient delay, but my small sample does not conclusively prove this will be the case. If there are concerns about the delay, then a multiplexer chip , 'OR gate' equivalent, or analog switch could be inserted in to the UART to dual MAX3221 path.This way the drivers do not have to be turned off.
Thank you. This answers my query and i would like to appreciate the prompt response and excellent technical support.
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