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ttl-rs232.htm
RS232 신호 변환회로
 RCL-1 RS232 Level Converter in a DB9 backshell Methods TTL to RS232 Use a TTL HI output for the RS232 GND: Steve Walz of armory.com says "Use a TTL HI output for the RS232 *GROUND*!! Then you get +/-5VDC levels to fake out the RS232. When TTL-GND goes HI, then a TTL-XMT LO looks like -5VDC and a "mark". Even sign problems go away in hardware! When TTL-GND goes LO, and TTL-XMT goes HI, it looks like +5VDC and a "space"! You can derive the two TTL signals from one off an inverter or inverter pair! Then magically, you have direct TTL to RS232 conversion!! M. Adam Davis points out that we should "Just *make sure* the power supply of the pic and/or the power supply of the other rs-232 device are not grounded. ie, my bench power supply has +5v and ground, but the ground is grounded to the house ground, SO is the computer's rs-232 port. I can't use this trick without isolation. This also means that any interaction with other external devices requires some thoughtful planning. " [Ed: this is only true if the devices confuse signal ground and chassis ground. Remember that ground is at DB25 pin 7, and not pin 1 Pin 1 is chassis ground, not signal ground.] Bob Ammerman [RAMMERMAN at PRODIGY.NET] of RAm Systems says: Here is a neat trick to get a 'push-pull' output from two PORT A pins: 1: define them as analog (ADCON1) 2: TRIS them as outputs. Now, when you BSF either of them the other will automatically be cleared. Andrew Warren says: Of course, if you can't configure the pins as analog inputs (because, for instance, the PIC you're using doesn't HAVE an A/D converter), you can do this: MOVLW 00000001B ;RA0 = 1, RA1 = 0. MOVWF PORTA ; MOVLW 00000011B XORWF PORTA ;Toggle RA0 and RA1. XORWF PORTA ;Toggle the two pins again. XORWF PORTA ;And again... Or, if those two pins are the only ones defined as outputs on that port: MOVLW 00000001B ;RA0 = 1, RA1 = 0. MOVWF PORTA ; COMF PORTA ;Toggle RA0 and RA1. COMF PORTA ;Again. COMF PORTA ;And again... Steal the -12 from the RX or other pins: Pin 2 on a peripheral connector (pin 3 on a host connector) will typically rest at -12 as driven by the external device. My Fluke 77/AN reads about -10 on pin 3 of the DB9 connected to my PC. Some people (Tom Engdahl at) http://www.hut.fi/Misc/Electronics/circuits/rspower.html have suggested using this fact to derive -12 for TX power. This makes sense as we normally are not RXing as we TX. How about adding a diode, resistor and big fat cap to provide a more or less steady source of -12v to drive pin 3 below zero when the NPN is biased off? Scenix did this on a demo modem board, http://www.piclist.com/techref/scenix/SX_Modem_demo.PDF 이 번역문은 ipstack.co.kr의 소유입니다, 임의의 복제 및 배포를 금합니다. 단 원저자, 원저자 사이트, 번역자, 번역자 사이트를 밝히면 배포가 가능합니다. 번역 : ipstack@ipstack.co.kr 날짜 : 23.Mar.2003. |
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