FPC cable for RS-485?
FPC cable for RS-485?
(OP)
I really wanted to search the posts before asking, but it seems the search engine is down, so please be kind if this has been asked before:
RS-485 bus would usually work best with twisted-pair 120 ohm cable. However, In a new application we are developing, a very flexible cable of this description doesn't exist (for a robotic application with a very small bend radius <1" for the cabling). In the past we have had success using FPC or flexible printed circuit cable for sensors and power and such, but not for a serial bus. Would anyone reccommend such a cable for RS-485 (5V bus)? The distances would be short, less than 8 feet total of bus length, and the bus drivers are slew-rate limited to speeds of about 500kbps. There would be approximateley 4 nodes on the bus, and the cable may be near a DC power cable.
We have not used RS-485 in a design before, so we just want to make sure we make the best choice among available options.
Thanks!
RS-485 bus would usually work best with twisted-pair 120 ohm cable. However, In a new application we are developing, a very flexible cable of this description doesn't exist (for a robotic application with a very small bend radius <1" for the cabling). In the past we have had success using FPC or flexible printed circuit cable for sensors and power and such, but not for a serial bus. Would anyone reccommend such a cable for RS-485 (5V bus)? The distances would be short, less than 8 feet total of bus length, and the bus drivers are slew-rate limited to speeds of about 500kbps. There would be approximateley 4 nodes on the bus, and the cable may be near a DC power cable.
We have not used RS-485 in a design before, so we just want to make sure we make the best choice among available options.
Thanks!





RE: FPC cable for RS-485?
Dan - Owner

http://www.Hi-TecDesigns.com
RE: FPC cable for RS-485?
I am also looking for other types of cables which are highly flexible with small bend radius, which we also use, but are not avaiable in twisted pair (with the same flexible properties). How would a three-conductor flexible cable (with the 3 wires twisted so as to give the cable its flexibility) work for RS485? One conductor for ground, but all three cables appear to be twisted in parallel, so I don't think its really a true twisted-pair. What do you think? I've been told a cable like this has impedance of 60ohms from one cable provider.
RE: FPC cable for RS-485?
RE: FPC cable for RS-485?
Two layers is the only way you can get twisted pair in FFC/FPC... so again, are you sure you need twisted pair? What's the noise environment? Any particular reason you can't use standard multi-conductor wire? A thin gauge would be almost as thin as a Parlex FFC, can be ganged for higher currents, and would have no problem meeting the <1" bend radius.
Dan - Owner

http://www.Hi-TecDesigns.com
RE: FPC cable for RS-485?
The environment is a multi-axis robotic arm. The cabling to the arm is snaked along its length, and will include 24V DC power, logic signals, and the RS485 bus, and possibly some stepper motor power cables as well. These most likely will be all in separate cables, but also likely bundled close together in the same cable chain. The cable chain keeps the cables together and formed properly for repeated bending/flexing as the arm moves. So as for noise, there is the possibility of stepper motor power cables, 24V DC power supplied to motor controllers embedded in the arm (attached to the rs485 bus), and the likelyhood of close proximity to some motors and/or solenoids.
I am still investigating the use of standard multi-conductor wire, similar to what we have used before, but the flex cable is more attractive for the reasons I mentioned in the previous messages.
We currently get long cables from Parlex (about 10 feet) and the cost is minimal, though the cable is standard.
RE: FPC cable for RS-485?
RE: FPC cable for RS-485?
RE: FPC cable for RS-485?
RE: FPC cable for RS-485?
Yes, the bending is constant, and the cable usally has a rating of number of bending cycles at the specified bending radius. A good flexible cable will have this number in the millions of bend cycles. IDC cable would not normally last this long, but I have seen the cable you describe.
RE: FPC cable for RS-485?
However, from a transmission-line point of view, the impedance between twisted and untwisted wires is little. Twisting allows the net RF field exposed to a balanced line to cancel. The differential driving also supresses any common mode sensitivity. Also, 115K baud is, in today's world, fairly slow.
You can try what was done on my project. Route a cable poorly. Cram the maximum baud you can through it. Verily you have no problem, and then back-off on the baud to provide additional margin on top of that.
RE: FPC cable for RS-485?
RE: FPC cable for RS-485?
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Dan - Owner

http://www.Hi-TecDesigns.com
RE: FPC cable for RS-485?
RE: FPC cable for RS-485?