Post by thread:[PIC] - Transistor Not Gate (Inverter) for UART Rx

I have a 18F1320 where the UART works goes to TTL fiber optic transmitter (Optek OPF1412T) and Receiver (Optek OPF 2412). Using a 74LS00 Nand gate on the receiver with a 560 ohm pullup on the one NAND input and other tied to 5 volt, everything works fine at 9600 baud. Prefer using transistor inverter. First tried (NPN) 2N2222A with :5 volt > 1K ohm > collector, Emitter > ground, and signal (from OPF 2412) > 4.7k ohm > Base. With Pic online, no joy, but remove the PIC and jumper TX > RX, OK. As I understand the 1320 (with schmidt) low level needs to be 0.2 volt or less, and I was getting 0.3 volt. Then tried (PNP) 2N3906 with : 5 volt > Emitter, Ground > 1K ohm to collector, and signal > 4.7K > base. Now the low is getting to 0.00 volts. Tried a 0.1 mfd capacitor between 5 volt and ground. The Tx > Rx jumper thing still works, but the 1320 should receive one of several characters and respond with a string or a character, which isn't happening. I also have a MAX232 on the circuit, the transmitter (from the 1320) always active, but a jumper to select either the RS-232 or fiber receiver so as not to drive the RX pin from 2 sources. The board is a PICPROTO18, the fiber optic goes to a 6' Tyco jumper cable (with loopback works fine) to a B&B FOSTC serial (either RS-232, Rs-485, but now RS-232) to fiber optic converter to a PC. Assuming I have some sort of timing issue with rise fall times, but I don't have a O-scope to troubleshoot. :) Thoughts??

Carl, My guess would be the 1k between the collector and the +5V line is too high to pull the line up fast enough - how long is the line or is there addtonal capacitance on it? 1k @ 5V is "only" 5mA . at 9600 bits/sec the risetime required is goiig to be better than 26uS (1/4 bit?) If the schmitt input threashold is 0.2V then Try dropping the 1k to 330 ohms or so. Or you could use a PNP/NPN pair to actively pull the line high. RP On 01/11/06, Carl Denk <spam_OUTcdenkTakeThisOuTalltel.net> wrote: {Quote hidden}> I have a 18F1320 where the UART works goes to TTL fiber optic > transmitter (Optek OPF1412T) and Receiver (Optek OPF 2412). Using a > 74LS00 Nand gate on the receiver with a 560 ohm pullup on the one NAND > input and other tied to 5 volt, everything works fine at 9600 baud. > Prefer using transistor inverter. First tried (NPN) 2N2222A with :5 volt > > 1K ohm > collector, Emitter > ground, and signal (from OPF 2412) > > 4.7k ohm > Base. With Pic online, no joy, but remove the PIC and jumper > TX > RX, OK. As I understand the 1320 (with schmidt) low level needs to > be 0.2 volt or less, and I was getting 0.3 volt. Then tried (PNP) 2N3906 > with : 5 volt > Emitter, Ground > 1K ohm to collector, and signal > 4.7K > > base. Now the low is getting to 0.00 volts. Tried a 0.1 mfd capacitor > between 5 volt and ground. The Tx > Rx jumper thing still works, but the > 1320 should receive one of several characters and respond with a string > or a character, which isn't happening. > > I also have a MAX232 on the circuit, the transmitter (from the 1320) > always active, but a jumper to select either the RS-232 or fiber > receiver so as not to drive the RX pin from 2 sources. > > The board is a PICPROTO18, the fiber optic goes to a 6' Tyco jumper > cable (with loopback works fine) to a B&B FOSTC serial (either RS-232, > Rs-485, but now RS-232) to fiber optic converter to a PC. > > Assuming I have some sort of timing issue with rise fall times, but I > don't have a O-scope to troubleshoot. :) > > Thoughts?? > -

Now the line between the pic and the breadboard that the transistor and resistors are on is roughly a foot apart. If I can get this working a little, I'll move the stuff to the PIC's board. I'll try dropping the resistor, someone else suggested 470 ohms, but at the moment the MAX232 quit working, that's my fall back, get that working, then move the DB-25 and connect to the fiber transceiver. Thanks. :) Richard Prosser wrote: {Quote hidden}> Carl, > My guess would be the 1k between the collector and the +5V line is too > high to pull the line up fast enough - how long is the line or is > there addtonal capacitance on it? 1k @ 5V is "only" 5mA . at 9600 > bits/sec the risetime required is goiig to be better than 26uS (1/4 > bit?) If the schmitt input threashold is 0.2V then > Try dropping the 1k to 330 ohms or so. Or you could use a PNP/NPN > pair to actively pull the line high. > > RP > > > On 01/11/06, Carl Denk <.....cdenkKILLspam@spam@alltel.net> wrote: > >> I have a 18F1320 where the UART works goes to TTL fiber optic >> transmitter (Optek OPF1412T) and Receiver (Optek OPF 2412). Using a >> 74LS00 Nand gate on the receiver with a 560 ohm pullup on the one NAND >> input and other tied to 5 volt, everything works fine at 9600 baud. >> Prefer using transistor inverter. First tried (NPN) 2N2222A with :5 volt >> > 1K ohm > collector, Emitter > ground, and signal (from OPF 2412) > >> 4.7k ohm > Base. With Pic online, no joy, but remove the PIC and jumper >> TX > RX, OK. As I understand the 1320 (with schmidt) low level needs to >> be 0.2 volt or less, and I was getting 0.3 volt. Then tried (PNP) 2N3906 >> with : 5 volt > Emitter, Ground > 1K ohm to collector, and signal > 4.7K >> > base. Now the low is getting to 0.00 volts. Tried a 0.1 mfd capacitor >> between 5 volt and ground. The Tx > Rx jumper thing still works, but the >> 1320 should receive one of several characters and respond with a string >> or a character, which isn't happening. >> >> I also have a MAX232 on the circuit, the transmitter (from the 1320) >> always active, but a jumper to select either the RS-232 or fiber >> receiver so as not to drive the RX pin from 2 sources. >> >> The board is a PICPROTO18, the fiber optic goes to a 6' Tyco jumper >> cable (with loopback works fine) to a B&B FOSTC serial (either RS-232, >> Rs-485, but now RS-232) to fiber optic converter to a PC. >> >> Assuming I have some sort of timing issue with rise fall times, but I >> don't have a O-scope to troubleshoot. :) >> >> Thoughts?? >> --

San Mateo, 31 oktober 2006. I'm not sure if this helps, but in my Drake 2 I used an RTL inverter to handle my RS-232 input. It should be somewhere in the schematic: http://people.zeelandnet.nl/whhofman/jen//PIC/drake2tvonly.png R7 is 27 kOhm, R8 is 1 kOhm, the transistor is a 2N4124. I've had successes with it up to 100kbps, although I generally use it at 2.4Kbps, so I'm not sure if those 100kbps speeds are reproducable. And yes, the circuit relies on the diodes inside the PICmicro to handle excesses. In a real circuit you might want to have additional diodes external of the PICmicro. Greetings, Maarten Hofman.

Thanks for the comments. I had 2 other replies that suggested R7 be 330 or 470 ohm. One's comment was to make sure the transistor was saturated, and current not that much. Since this runs off 120 volt and not battery, a little extra power not a problem. The Pic quit even on the RS-232, have had problems with that before, don't know why, but in the morning I'll take a good look at things. :) Maarten Hofman wrote: {Quote hidden}> San Mateo, 31 oktober 2006. > > I'm not sure if this helps, but in my Drake 2 I used an RTL inverter to > handle my RS-232 input. It should be somewhere in the schematic: > > http://people.zeelandnet.nl/whhofman/jen//PIC/drake2tvonly.png > > R7 is 27 kOhm, R8 is 1 kOhm, the transistor is a 2N4124. I've had successes > with it up to 100kbps, although I generally use it at 2.4Kbps, so I'm not > sure if those 100kbps speeds are reproducable. > > And yes, the circuit relies on the diodes inside the PICmicro to handle > excesses. In a real circuit you might want to have additional diodes > external of the PICmicro. > > Greetings, > Maarten Hofman. >