RS232 circuit

[flemmard]

Hi all,
As per attached diagram , it's said that it can pull up 5v from PIC to 13v for Rs232 connection.But when i referred the MAX3222 datasheet, it's said that it can only pull up 3.5v to 5v.Then in such case , how it works?

Any help, please
Thanks

 

[ericgibbs]

Hi all,
As per attached diagram , it's said that it can pull up 5v from PIC to 13v for Rs232 connection.But when i referred the MAX3222 datasheet, it's said that it can only pull up 3.5v to 5v.Then in such case , how it works?

Any help, please
Thanks

If I read it correctly the datasheet says the MAX will operate from a supply of +3.5v thru +5v and the internal voltage pumps will generate about +/5.5V to suit RS232.

 

[mvs sarma]

If I read it correctly the datasheet says the MAX will operate from a supply of +3.5v thru +5v and the internal voltage pumps will generate about +/5.5V to suit RS232.

appended here are the main electrical characteristics
VCC...........................................................................-0.3V to +6V
V+ (Note 1) ...............................................................-0.3V to +7V
V- (Note 1) ................................................................+0.3V to -7V
V+ + V- (Note 1)...................................................................+13V
Input Voltages
T_IN, SHDN, EN ...................................................-0.3V to +6V
MBAUD...................................................-0.3V to (VCC + 0.3V)
R_IN .................................................................................±25V
Output Voltages
T_OUT...........................................................................±13.2V
R_OUT....................................................-0.3V to (VCC + 0.3V)
Short-Circuit Duration
T_OUT ....................................................................Continuous

There appears a controversy.

(v+ + v-) diff between v- to v= pins can't excced 13V even though individually thay can reach 7V
if so, how to interpret the output voltages T_ out and R_ out as +/- 13V

 

[blueroomelectronics]

The MAX3222 uses a charge pump to almost double the VCC both + & -
Internal low drop diodes take a little so your maximum output voltage will always be slightly less than VCC*2 (approx 0.5V less)
So with a 3V supply you'll get +5.5V & -5.5V which is within the RS232 spec (even though it's on the low side)

 

[Sceadwian]

RS232 specs say the transmitter is supposed to generate -12/+12 but the receiver needs to be able to trigger at -3/+3 so there's a lot of wiggle room on the practical side.

 

[Papabravo]

RS232 specs say the transmitter is supposed to generate -12/+12 but the receiver needs to be able to trigger at -3/+3 so there's a lot of wiggle room on the practical side.

I think this is not quite the correct story. It is true -3/+3 is the no man's land of invalid signals. You can transition through that range , but you can't stay there. As for the other levels, they can be up to and including -25/+25. Receivers need to be aware of this and survive. The 12V selection was just one of the many possible ones made by device manufacturers. Check out the specs for the venerable MC1488 driver and MC1489 Receiver. Those parts required multiple supplies and predated the MAX232 which introduced the single supply transceiver with a charge pump and an inverter.

 

[Leftyretro]

I think this is not quite the correct story. It is true -3/+3 is the no man's land of invalid signals. You can transition through that range , but you can't stay there. As for the other levels, they can be up to and including -25/+25. Receivers need to be aware of this and survive. The 12V selection was just one of the many possible ones made by device manufacturers. Check out the specs for the venerable MC1488 driver and MC1489 Receiver. Those parts required multiple supplies and predated the MAX232 which introduced the single supply transceiver with a charge pump and an inverter.

You state correctly from the standard that I recall, however.....

Having worked with RS-232 sense the the early 70s, I have to say it's the most abused standard I have ever dealt with It's amazing how many firms and devices actually get their serial devices to function with the liberties they take. Take for example the serial download circuit attached that connects the Picaxe series of micro controllers (PIC based). Note that the serial output from the Picaxe to the PC only switches between 0v and +5. By not passing through to -3v or more negative for the 'mark' state the circuit should not work reliably according to the standard, but in reality it seems to work very well as it's the only method to download a program to a Picaxe. There have been problems noted by some using laptops but that is most likely because of the PC to Picaxe direction not going as high positive as most desk top PCs. I've read that many laptops only switch at +/- 5 volts.

Lefty

 

[ericgibbs]

Lefty,

This has been my experience also.

The main limitations of course are the cable lengths and environmental em noise levels at these lower voltage levels.

Also, at the lower levels of +/-5V the em noise radiation is reduced.

 

[Leftyretro]

Lefty,

This has been my experience also.

The main limitations of course are the cable lengths and environmental em noise levels at these lower voltage levels.

Also, at the lower levels of +/-5V the em noise radiation is reduced.

You are correct. But it is amazing what one can get away with. In the 80s as a tech I helped install a lot (hundreds!) of CRT and printers in control houses at a oil refinery I worked at. We use to make our own serial cables using 24 conductor flat unshielded ribbon cables and IDC connectors. Many of these cable runs worked out to be 100-150 feet and running above ceiling panels, behind large control panels, etc. Very noisy environment and we usually were doing 9600 baud or better. Unshielded cable, long runs, noisy conditions, etc...worst case type stuff...but they always seemed to work well I think the standard suggests a 50ft max cable distance. However in those days +/- 12 to 15 volts was the normal switching voltages.

Lefty

 

[blueroomelectronics]

I agree with Lefty, RS232 has got to be the most abused standard ever, I think RS422 & RS485 are so much more practical and work like a charm at 5V. Long range, multidrop...