RS485 weirdness. Advice please?!

[Guest Rockford]

OK, I have a VCL camera connected and terminating one end of an RS485 bus. I have an RS232-RS485 converter on the DVR to control the camera. The DVR should be terminated, but the converter has no option for that. What I'd like to do is to terminate the bus after the DVR using a 120ohm resistor.

With the DVR at one end and the VCL camera at the other, the RS485 bus is reading an impedence of 145ohms! I tried to put a 120ohm resistor on the bus AFTER the DVR, but this left the bus impedence unchanged... I mean, what on Earth is that suppost to mean??!

Anyway, to add N to X, if I use an oscilloscope, there's a lot of noise on the RS485 bus and I cannot read a signal when I transmit telemetry data. Using the diagnosis tool on the DVR, it is definately sending the signals to the bus.

lastly, to further complicate, if I use a different DVR with an on-board VCL controller (terminated by default), it works perfectly!

The problem is that I don't know enough about RS485 to troubleshoot this issue further...

Can anyone offer me some useful advice? Many thanks in advance!

[Guest]

broken converter?

did you test the converter?

if you have dvr with built in converter just connect that other converter to that and rs232 to your computer com port and listen with hyperterminal.

[Guest Gordon]

Hi there, here is what Intersil have to say about RS458:-

Proper termination is imperative, when using the 15Mbps

devices, to minimize reflections. Short networks using the

250kbps versions need not be terminated, but, terminations

are recommended unless power dissipation is an overriding

concern. In point-to-point, or point-to-multipoint (single driver

on bus) networks, the main cable should be terminated in its

characteristic impedance (typically 120Ω) at the end farthest

from the driver. In multi-receiver applications, stubs

connecting receivers to the main cable should be kept as

short as possible. Multipoint (multi-driver) systems require

that the main cable be terminated in its characteristic

impedance at both ends. Stubs connecting a transceiver to

the main cable should be kept as short as possible.

Remember that impedance and resistance are *not* the same thing. Also

how are you 'scoping the signal ? UTP has built in noise canceling

due to the the fact that both conductors pick up the same amount of

noise and the differential reciever cancels this mutual noise. You will

need to 'scope both A and B in diff mode on your scope.

Hope this helps

[Guest]

yepp, Z and R are different but when terminating RS485 R120 is fine.