| ??? 02/19/09 23:29 Read: times |
#162580 - daisy chain Responding to: ???'s previous message |
Neil Kurzman said:
"a terminating resistor of 120 Ohms" There should be 1 on either side of the bus.
Are they daisy chained or in a star configuration? I thought I could get away with removing the terminating resistor for the transmitter, but for the receivers they all should have the 120 Ohms resistor. Any way when I connected the terminating resistor anywhere on the bus I got no data received on the receivers. I can't remember what the waveform on the scope looked like but will double check tomorrow. Michael Karas said:
Did you put the resistor across between the (A) and (B) wires of the bus? yes sir exactly as the datasheet quoted. Michael Karas said: What do you mean by not having en adequate VCC and GND return to the driver chip? I connected the VCC and GND pins to the 5v power supply (LM7805) normally like any other chip.
You should check with a scope as to what is happening with the signal level when you insert and remove the resistor. Maybe you have a problem of not having an adequate VCC and GND return to the driver chip in the master and the signal level is suffering greatly when you add the 120 ohm load to the bus. I made the connection as follows: +12v power supply feeds the master board through LM7805 regulator. Then same power supply goes with Y and Z lines of transmitter to the receiver board where the +12V power supply goes to LM7805 regulator in the slave circuit. Should there be a seperate GND pin to the Y Z lines or would the power supply return line suffice? Erik Malund said:
3 resistor termination or "zero safe" transceivers.
if your transcievers are not "zero safe" any "one resistor" termination can upset the apple cart (or sometimes righten it) anyhow "one resistor" termination is not sufficient for transcievers that are not "zero safe". Erik I only tested it with one slave with a 100m cable and a 0.1m cable with same results. I will try adding more slaves tomorrow and see what happens. Kai Klaas said: Surely the signal level gets very low to upset the serial communications.
What does your scope tell about the corresponding signal levels? Everything fine?
Kai I have another question, I placed the LM7805 on the metal Chassis of the clock case and made wires about 10cm from the regulator to the board. is this ok or would it introduce noise into the circuit and I should make the regulator leads as short as possible? |
| Topic | Author | Date |
| RS485 problem | 01/01/70 00:00 | |
| Just 1? | 01/01/70 00:00 | |
| The 120 ohm resistor | 01/01/70 00:00 | |
| VCC = 5V | 01/01/70 00:00 | |
| you need | 01/01/70 00:00 | |
| Scope? | 01/01/70 00:00 | |
| daisy chain | 01/01/70 00:00 | |
| Some hints... | 01/01/70 00:00 | |
| 100 meters | 01/01/70 00:00 | |
| 7 clocks | 01/01/70 00:00 | |
| Topology | 01/01/70 00:00 | |
| RE: topology | 01/01/70 00:00 | |
| don't | 01/01/70 00:00 | |
| Yes, don't! | 01/01/70 00:00 | |
| Getting there | 01/01/70 00:00 | |
| So, what was the cause of failure? | 01/01/70 00:00 | |
| not yet | 01/01/70 00:00 | |
| with additional components | 01/01/70 00:00 | |
| Where do you have this circuit from? | 01/01/70 00:00 | |
| from my Mentor | 01/01/70 00:00 | |
| Ben's site | 01/01/70 00:00 | |
| Mission Accomplished | 01/01/70 00:00 | |
| I'm not sure I agree | 01/01/70 00:00 | |
| exactly | 01/01/70 00:00 | |
| Maybe I misunderstood your earlier post | 01/01/70 00:00 | |
no | 01/01/70 00:00 | |
| yes | 01/01/70 00:00 | |
| then how could you ask .. | 01/01/70 00:00 | |
| With noise+etc reduction | 01/01/70 00:00 | |
| email updated | 01/01/70 00:00 | |
| here's info's........ | 01/01/70 00:00 | |
| That means you get | 01/01/70 00:00 | |
| Only when seeing nosie? | 01/01/70 00:00 | |
| Not exactly but as per requirement | 01/01/70 00:00 | |
| CAT5 ethernet cable | 01/01/70 00:00 | |
| 100 Ohm | 01/01/70 00:00 | |
| More details here | 01/01/70 00:00 | |
| did you check the FAQ? | 01/01/70 00:00 |