I see sellers listing different versions of the max485 chip. Does it matter? I want to make sure I don't buy the wrong thing. So far I have come across a:
I can't really tell from the spec sheets what the difference is. There also seems to be huge disparity in prices. Like this vendor has 50 chips for $9. That's allot of chips!
The letters after a part number normally specify the package they are in and the temprature range they can operate at. Sometimes it will also denote the speed.
Are they that cheap? They can be if they are surplus, out of spec devices from the reject bin, or have not been payed for. Bulk prices are often quite cheap and $0.20 each for that type of chip looks to me like it is the 100,000 up price.
Thanks for the clarification. They all look the same in pictures so it sounds like it won't matter much. For $0.20 I guess I can't really go wrong. Hopefully these chips will allow me to daisy chain my tlc5940s out to a usable distance.
Hopefully these chips will allow me to daisy chain my tlc5940s out to a usable distance.
Possible not, these are slew rate limited drivers. The slew rate is the rate of change in voltage. That means the rise and fall time of the signals is deliberately slowed down. This is fine when driving serial coms data but it will limit the top frequency you can send down them. Check first what the top frequency of the TLC5940 needs to be, this will be the clock. Then calculate what the maximum frequency of these drivers is.
My plan was not to link the TLCs directly with the max485s (sorry I realize my post was vague). I plan to have an atmega328 at each location which will interpret the serial commands sent to it by the arduino master through the max485, and then it would in turn tell the TLC at its location which LEDs to turn on. So each node will contain a atmega328, a TLC5940, and a max485 plus (crystal, resistors, pins etc).
I only need to tell the slaves what to do and do not need them to talk back to the master so half duplex should be fine right? The whole reason for me going for this node based approach to begin with is to cut down on the wiring needed.
Please tell me I have not misunderstood this whole thing, I have a pile of chips on order
Thanks for you help guys, the bit about the clock speeds really shed some light on a bunch of blank spots I had in the way these pieces interact.
Grumpy_Mike:
The letters after a part number normally specify the package they are in and the temprature range they can operate at. Sometimes it will also denote the speed.
Are they that cheap? They can be if they are surplus, out of spec devices from the reject bin, or have not been payed for. Bulk prices are often quite cheap and $0.20 each for that type of chip looks to me like it is the 100,000 up price.
Incredibly cheap prices are a good sign that something is not legit.
Buy your parts from digikey, mouser, someone that has traceability back to the manufacturer.
Digikey, Mouser, Avnet, Newark, all show this part as a $2+ price/chip in 50-lots.
Or take your chances. A real company would not buy parts from e-bay. A hobbyist certainly can. At worst, you lose some time debugging & stuff, finally determine you got hosed and go get real parts.
I'm having the opposite problem with counterfeit Chinese devices.
I bought a lot of 10 devices Max488 (which offer full-duplex 4-wire RS485 link) from AliBaba
At $0.30 per device, that's a steal.
However for the life of me, I couldn't get a proper conversion from the differential RS485 signal to a single-ended TTL signal. I could observe the input signals being clamped down, as soon as I attached them to this Max488 device. Endless pondering of the Maxim datasheet led me to doubt whether I was using the proper pinout. Maxim's half-duplex devices seem to use one generic pin diagram, while the full-duplex devices share another generic pin diagram. I adjusted the wiring on my alleged full-duplex device to the pin diagram of the half-duplex devices, and all of a sudden my test worked.
So I'm concluding that this sale is really a steal.
I have gotten 10 devices which are labeled as Max488, but which I guess are actually Max485.
CrossRoads:
Incredibly cheap prices are a good sign that something is not legit.
Buy your parts from digikey, mouser, someone that has traceability back to the manufacturer.
Digikey, Mouser, Avnet, Newark, all show this part as a $2+ price/chip in 50-lots.
Or take your chances. A real company would not buy parts from e-bay. A hobbyist certainly can. At worst, you lose some time debugging & stuff, finally determine you got hosed and go get real parts.
I have a customer or two, BIG corporations, that are building such old devices that Ebay and other such is the only source for them. And usually the chips then get don't work properly.
If we can't source components form the usual distributors, or our one proven gray market source, the customer must supply them.
And no, they won't redesign the boards. No one there knows where the specs are stored.