Sorry for my limited English ability and Technical ability.
It is very common in our company that, the engineers don't care about (or don't know) the termination resistors on a CAN-Bus. They just simply put one 60-Ohm resistor between two or several CAN-Devices (mainly for testing purpose). I know this is not standard, and I don't like this workaround. I tried to figure out why such a workaround is bad, but failed.
It seems that many engineers use such a workaround. ==============================> www.microchip.com/.../m177894-print.aspx a single 60 ohm termination for lab testing when bus is only a few feet (probably not allowed, but it works).
www.microchip.com/.../m144034-print.aspx If you don't have two 120-ohm resistors, you can accomplish the termination (on a small network) with a single 60 ohm resistor(120 in parallel with 120 is 60) or anything close 55-65 is fine. <==============================
I found a document saying this workaround is a mistake, but it does not provide why. ==============================> www.scribd.com/.../SAE-J1939 The bus is linear and should be terminated with 120-Ohm resistors at either end. It is a common mistake to use one 60-Ohm resistor instead of two 120-Ohm resistors. This does not work correctly, however. <==============================
What is the disadvantage/mistake, if we use one 60-Ohm resistor instead of two 120-Ohm termination resistors on a CAN-Bus, assuming the communication distance is not very long and only 3-5 CAN-Nodes (Devices) involved? In another words, when will such a workaround fail to work?
-- It is very difficult to find another job in southern Taiwan. So I am still struggle with idiotic things, and busy in urgent projects.
now it's CAN, it used to be RS485 these questions were about. However there is only one and the same answer:
The only way to ensure proper transmission is proper termination
Ever so many have "gotten by" without proper termination till, some day, someone intalled the equipment in a different place than the testbench.
what value does (mainly for testing purpose) have, if you are not testing under the conditions that exist for shipped equipment.
Erik
Does one 60-Ohm resistor provide the same funtionality as two 120-Ohm termination resistors?
No. Because you'll have reflections on your cable that way. On short cables you may not notice any difference, but if you use a sufficiently long run of cable (> 50 meters, roughly), and use a scope fast enough to measure timing differences below 100 ns, you'll see the difference.
In a nutshell, a single termination resistor is always wrong --- the exact gravity of the inevitable consequences depends on your network's propagation delay compared to the bit time.
Many thanks for the helps.
"verifies explicitly what reflections they have in the cable" Can this be done by an oscilloscope? I don't think we know how to do this correctly.
Our baudrate is 500 kBit/s. Does one 60-Ohm resistor provide the same funtionality as two 120-Ohm termination resistors?
But when the debate is about 120 ohm or 60 om termination, I think we can rule out your CAN-over-fiber or sing-ewire PHY from the debate.
For short cables, you can often get by without any termination unless the baudrate is very high.
Close, but not quite correct. Whether working without a terminator is possible at all depends primarily on which of several physical layers for CAN is in use; speed only comes into play because different PHYs have different ranges of allowed baud rates.
There's the single-wire PHY used by GM that has no termination at all. There's the "low-speed", fault-tolerant CAN PHY which usually doesn't need a terminator, but maxes out at 125 kBit/s. And then there's "high-speed" differential-only PHY, which will not work at all without some kind of termination, but goes up to 1 MBit/s in return. Not to mention CAN-over-fiber.
Note that the terminating resistors do DC-load the signal.
Having a terminating resistor matching the characteristic impedance is for AC functionality - making sure there are no signal bounces.
For a very short cable, a single 120 ohm resistor would normally work. But may not be needed depending on propagation delay compared to your transfer rate.
How about your engineers verifies explicitly what reflections they have in the cable? No need to have extra loads if there aren't really any problems with reflections - less load is less power wasted.