| ??? 06/13/06 03:32 Modified: 06/13/06 03:42 Read: times |
#118185 - Mild shocks... Responding to: ???'s previous message |
Ganeshan said:
I have a board (B1) that communicates with another equipment (E1) over RS-485. The RS485 interface circuit consists of MAXIM MAX485 I highly recommend you to use a slew rate limited RS485 driver, like LT1785. It forgives slight mistakes with the termination and the cable. Ganeshan said:
Cable length between E1 and B1 is about 1.5 metres.
Problem remains independent of terminating resistance (120E) (enabled/disabled) across RS485 D+ and D- pins .. Cable between B1 and E1 has three wires - D+,D-,Gnd. Behaviour is same ,whether I connect this 3rd Gnd wire or not. Some MUSTS: 1. You must always transmit both signal lines AND signal ground! 2. The two signal lines (+D and -D in your example) must be transmitted via a twisted pair. 3. Characteristic impedance of this tiwsted pair and the added termination resistances must match. In your case a twisted pair showing 120Ohm is needed. 4. Termination resistances must be present at both sides of RS485 cable, means directly at the driver and directly at the reciever! Again, these MUSTS are demanding when you use a MAX485, which is a driver without slew rate control. With a LT1785 your application might work even without using a twisted pair and without having a perfect match between cable termination and cable impedance, although this is not recommended. Ganeshan said:
But when I connect this B1 to a standard RS232/485 converter and connect to PC, B1 is able to send messages. (I have written a E1 simulator software on PC) - so as such there is no problem with MAX485 also. Then I connect B1 back to E1. Communication still does not take palce. Bravo! Well done! Which indicates, that it's not the RS485 driver causing problems. What's wrong with the E1 we cannot analyse unless you tell us more about the E1... Gansehan said:
I can't doubt E1 because it is supposed to be a proven equipment, in production. But you should take this into consideration as your test demonstrated that the MAX485 works as intended. (Which is not a proof, of course!) Ganeshan said:
Voltage between Earth and Neutral is of the order of tens of volts. E1 and B1 are both powered from a Spike buster which is connected to a single AC socket on the wall. Is the problem related to earthing not being of good quality?
.. When I touch E1 itself (E1 has a metal casing) it does not give any 'mild shock' As long as Neutral isn't connected to signal ground of E1, I cannot see any reason, why this potential difference should cause a problem, although it's not recommended to have such a big discrepancy between Neutral and Protection Earth (PE). Your earthing is fine if you don't get mild shocks when metal casing of E1 is connected to Protection Earth (your 3rd pin of 230V) at the same time. Ganeshan said:
3rd pin (Earth) of 230V cable is connected to E point on PCB and transformer body. E point is not connected to power supply Gnd. There should at least be a cap of 10...100nF between signal ground and "point E", resulting in a so called "soft earthing". Ganeshan said:
Occassionally I get a 'mild Shock' when I touch this DB9. (Not every time), that too only when E1 is connected. If I disconnect E1 then this 'mild shock' is not observed. When I touch E1 itself (E1 has a metal casing) it does not give any 'mild shock'. So, this means, you get a 'mild shock' when you touch signal ground of E1? This can be harmless: Big interwinding capacitance between primary and secondary winding of mains transformer, for instance, can inject "lots" of mains voltage onto secondary side of transformer, means signal ground. A simple 100nF cap between signal ground and Protection Earth can suppress this effect. Take care, this mild shock can also be an indication for a dangerous malfunction in the power supply, which can hurt or even kill you!! So, make the following checks: Disconnect E1 from mains and from B1, connect an Ohm meter between signal ground and metal housing of E1. What does the meter show? Connect a cap meter in the same manner. What does it show? Connect a diode meter in the same manner. What does it show? Connect the cap meter also between mains terminals of E1 (with E1 being disconnected from mains, of course!) and signal ground. What does it show? What does an Ohm meter show, being connected in the same manner? Tell us more about device E1. And find out the reason of "mild shocks"!!!!!!! Kai |
| Topic | Author | Date |
| Bad MAX 485 IC ? | 01/01/70 00:00 | |
| Voltage between Earth and Neutral.. | 01/01/70 00:00 | |
| also | 01/01/70 00:00 | |
| This may be one reason | 01/01/70 00:00 | |
| What's a DB9? | 01/01/70 00:00 | |
| Correction . DE9 - not DB9 | 01/01/70 00:00 | |
| Earthed Shell to the shield of cable | 01/01/70 00:00 | |
| Mild shocks... | 01/01/70 00:00 | |
| E1 is a pump controller | 01/01/70 00:00 | |
| Open "E1" and have a look at | 01/01/70 00:00 | |
| you missed one | 01/01/70 00:00 | |
| Only one reciever | 01/01/70 00:00 | |
| Yes. Only one receiver | 01/01/70 00:00 | |
all uC developers should be Boy Scouts | 01/01/70 00:00 |