Post by thread:MAX232, L78S05 and serial port power

Does anyone know if it's safe to draw power from DTR with a circuit made of L78S05, MAX232 and (of course) 16F84. I think the power requirements are <5 mA. The communication speed is 1200 baud. The only function of the circuit is to send data between the PC and the PIC. When I use an external power source instead of DTR and try to measure the current drawn, my multimeter behaves strangely (probably a fault in the multimeter). I apologize if this topic has already been discussed. I just lost all of my PICLIST messages. Best regards, Johann Geir Jonsson

> Does anyone know if it's safe to draw power from DTR with a circuit made of > L78S05, MAX232 and (of course) 16F84. "Safe" as in "can't damage the RS232 port": Yes. RS232 ports are designed to tolerate shortcircuits between any of their pins. Most RS232 port drivers are damaged by ESD events, which CAN happen at insertion of ANY equipment. "Safe" as in "works correctly with all RS232 ports": No. Simple reason: The MAX232 is supposed to talk to the RS232 port correctly. To generate the weakest allowed signals, it needs minimum X mA at 5V. X is the bare minimum to comply with RS232 specs. Confirmed? Now, the host RS232 port is allowed TOO to generate the weakest signals that comply with the SAME specs. Imagine, the PC designer has chosen a MAX232, and supplies it with exactly X mA at 5V. This is not the most common situation, but perfectly legal with the RS232 specs. Confirmed? Finally, the X mA current drive the host MAX232, the RS232 cable, your 78x05, and your PIC. What is LEFT of that current is surely less than X, isn't it? Your MAX232 will receive X-Y mA and NOT comply with RS232 specs anymore. If a RS232-powered MAX232 would give a valid RS232 port, I could use THAT to run another MAX232, and hey why not run another MAX from that, and ... It would give a nice 8-way MIDI interface with no power supply required! Reminds me of that perpetuum mobile thing. You should change your design to require less power! For example, use an LP2950-CZ50 as regulator, and a low frequency XTAL at the PIC. Put it asleep where possible. For the RS232 side, use a discrete transistor or FET interface, which either shorts the RX line or DTR to TX. Program the host software to put DTR+RTS high, and RX low (half duplex). This will work on many ports, but still NOT ALL. Or, simply use a power supply. Then you are perfectly in-spec!

part 0 937 bytes content-type:text/plain ----- Original Message ----- From: J—hann Geir J—nsson <spam_OUTjohannTakeThisOuTMB.IS> To: <.....PICLISTKILLspam@spam@MITVMA.MIT.EDU> Sent: 10 Haziran 1999 Perßembe 13:34 Subject: MAX232, L78S05 and serial port power > Does anyone know if it's safe to draw power from DTR with a circuit made of > L78S05, MAX232 and (of course) 16F84. I think the power requirements are <5 > mA. The communication speed is 1200 baud. The only function of the circuit > is to send data between the PC and the PIC. When I use an external power > source instead of DTR and try to measure the current drawn, my multimeter > behaves strangely (probably a fault in the multimeter). I apologize if this > topic has already been discussed. I just lost all of my PICLIST messages. > > Best regards, > Johann Geir Jonsson I hope attachment text help you. Attachment converted: wonderland:serialpower.txt (TEXT/CSOm) (0000890D)

Van: Nihat DAGDEMIR <showKILLspamAGT.COM.TR> Aan: .....PICLISTKILLspam.....MITVMA.MIT.EDU Onderwerp: Re: MAX232, L78S05 and serial port power Datum: zondag 31 mei 1998 1:19 <Big Snip> > I hope attachment text help you. Hello Nihat, I'm not sure, but it looks like that the '+' -char is used to signal connections as well as (non-conductive) crossings ... In the schematic DTR & RTS (as well as DSR, CTS & DCD) seem to be connected to each other. I don't think that's what you wanted. Maybe you could draw connections with the '+' -char, and replace the crossings with something like this: --|-- or | --- | Greetz, Rudy Wieser

J—hann Geir J—nsson wrote: > Does anyone know if it's safe to draw power from DTR with a circuit > made of L78S05, MAX232 and (of course) 16F84. I think the power > requirements are <5 mA. The communication speed is 1200 baud. The > only function of the circuit is to send data between the PC and the > PIC. An old "RS-232 FAQ". As previously noted (Marc Heuler), you don't use a 78L05 followed by a MAX232 as you simply lose efficiency at every step. If you want maximum efficiency, which is to say, to use every little bit of available current from the port or to operate from the weakest possible port driver, then: 1} You derive your +5V from every available RS-232 line through a diode i.e., TXD, RTS, DTR; three diodes directly from these lines to the +5V rail. 2} If you *must* loopback RTS to CTS and/ or DTR to DSR and DCD, then do so via 1k or 2k resistors to minimize loading by the receivers. 3} You regulate with a shunt Zener (5.1V) and a large capacitor (220µF) across the +5V supply. RS-232 is current-limited by definition so no problems here. If you *really* need a reference voltage, use a separate reference IC. 4} The most efficient driver for the TXD line, and protection on the RXD line is a 74HC14 gate. You can use the PIC directly if you want, but the gate is likely cheaper and easier to replace (in the field!). Using transistors is counter-productive as you lose current in the pull-up resistors, unless you also wish to use a negative supply derived from another three diodes to provide negative swing to outgoing data. Protective resistors in (incoming) TXD of about 33k ohm and (outgoing) RXD of about 220 ohm. 5} If you do *not* drive CTS, DSR, DCD and/ or RI, ground them to avoid spurious Line Status interrupts fouling up terminal routines on the PC. -- Cheers, Paul B.

At 09:57 AM 6/16/99 PDT, you wrote: > 5} If you do *not* drive CTS, DSR, DCD and/ or RI, ground them to avoid > spurious Line Status interrupts fouling up terminal routines on the PC. > >Doesn't that leave them smack in the middle of the "illegal" voltage >range for rs232 ? Possible--but some chips such as the MAXIM's that have auto-shutdown look for "no valid signals" on the input lines as the trigger for going to low power mode. Ground or zero volts in is "no valid signal" and should be what is seen if the RS232 lines are unplugged. Kelly **************************************************************************** ******** All legitimate attachments to this email will be clearly identified in the text. William K. Borsum, P.E. OEM Dataloggers and Instrumentation Systems <borsumspam_OUTdascor.com> & <http://www.dascor.com>

William Chops Westfield wrote: > Doesn't that leave them smack in the middle of the "illegal" voltage > range for rs232 ? I was referring to the common approximation of using the direct CMOS logic of 0 to 5V to drive the RS-232 line. For short lines (e.g., mice, other serial port interface attachments) this works fine because all RS-232 interface chips *actually* have a threshold of 2.5 volts. As to the alternatives of supplying the 5V logic, I was referring to a procedure to *maximise* available supply current. You don't get maximum supply current at 12V from the driver, you get it at 5V. You get it from RTS and DTS. TXD idles *negative* so it isn't useful, except as a source of negative voltage to send data back out (on RXD). There are many more exotic schemes, I was describing a simple and durable one (which depends on ports being initialized: RTS and DTR active. Some mice do funny things here). -- Cheers, Paul B.

Someone wrote: > 5} If you do *not* drive CTS, DSR, DCD and/ or RI, ground them to avoid > spurious Line Status interrupts fouling up terminal routines on the PC. BillW asks: > Doesn't that leave them smack in the middle of the "illegal" voltage > range for rs232 ? Yes. However, the EIA-232 receivers have to do something reasonable when there's no voltage potential between its input and signal ground, as in the case where that particular signal is not connected at all, or is mistakenly (or intentionally) wired to ground. To deal with this, they normally set the receiver threshold for the line inactive state to a little less than 0V. A grounded signal is thus interpreted as inactive, just as a signal in the proper 3V to 25V range would be. Of course, it's somewhat bad practice to count on this behavior; it is possible that some EIA-232 receiver circuits could be implemented less cleverly and produce an indeterminate output. If possible, it's better to tie unused status lines to an EIA-232 driver with an appropriate output. Note that I am *not* suggesting that unused status lines be tied directly to a low-impedance voltage source such as a +5V or +12V supply rail; that is a very bad idea since EIA-232 requires current limiting and short-circuit protection. In practice, grounding the status lines may be better than letting them float. For a well-designed EIA-232 receiver there would be no noticable difference; in a PC using some cheap knock-off of the 1489, who knows.

I wrote about EIA-232 receivers: > To deal with this, they normally set the > receiver threshold for the line inactive state to a little less than 0V. > A grounded signal is thus interpreted as inactive, just as a signal in > the proper 3V to 25V range would be. Oops, I was tripped up again by the inverting nature of EIA-232 transmitters and receivers. The inactive state of an EIA-232 line is negative, and the common receivers set the threshold above ground, so that a 0V input is considered inactive.