Converting SPI lines to diff. signals, using RS tranceivers

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Hi,

Say one wants to connect a graphical display with SPI interface to a µC from a certain distance (max 3 meters), could this be done using RS232(or 485) high speed transceivers, by just connecting the relevant CLK, CS, DIN(or MOSI) and power lines(or a total of five wires)?

The advantage over using a traditional serial connection, is the lack of an extra µC needed on the display side.

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If you don't need the MISO line I think this should be feasible. I would use RS485.

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Thanks, I also thought using RS485 transceivers over their RS232 brothers, for their greater throughput.

Regarding the MISO line, most displays with optional SPI interface, do not bring out the MISO line, only in parallel mode one can read back the internal display registers (as far as I know of).

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I agree on RS485 chips. At SPI data rates, speed should not be an issue. Fixed direction would mean not having to deal with the transmit enable. The problem would be the MISO line, but if that is not used, then no problem.

However, if thee is reason to change the data line to differential, then you probably ought to do the clock, also. Then, what is the point with these two without also the select line. It starts to get expensive, both for cost and power. And lots of wires.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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CS, CLK, MOSI, that's 6 wires plus 2 for power and ground, for a total of 8. You could use plain UTP for that.

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Well I'm wrong, I meant using RS232 transceivers like the max 232, and thus not using differential lines (I mixed RS485 with half- and full-duplex topology, where the first one only uses two wires for RX and TX), thus using one wire for each signal, with a total of five wires for CS, CLK, MOSI and power.

The distance of 3 meter should be ok for the RS232 transceivers, using straight TTL/CMOS signal levels is not an option, I think the limitation will be speed.

What do you guys think?

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Yes, you can use RS23 with a serious caveat: ONLY ONE LOAD!

Almost all RS232 receivers have a built-in load resistor. A transmitter is designed to drive only ONE of these loads. So, if you have only one display, then its fine. If you need to connect several devices, then not good.

You would be better off using a standard CMOS line driver and a standard CMOS gate as a receiver, Then, you install your own terminating resistor at the far end of the line.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Thanks Jim, it's just to add a remotely placed display as an option to the controller, thus only one display will ever be attached to this side of the lines.

Didn’t know that standard CMOS line drivers and receivers could be(are) used to that effect, but now you talk about it, it sounds logic(CMOS in this case :) ).

PS: on a side note, is it my PC or are you people also having some latency when typing text in the forum editor? Didn't had that before...

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No real typing latency problem, here. People have commented about the problem from time to time.

I would use an inverting line driver and an inverting gate as a receiver. The receiver also helps to protect the display pin from nasty ESD things. These would be a lot smaller and less expensive than RS232 (plus, you don't need all those caps).

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Quote:
PS: on a side note, is it my PC or are you people also having some latency when typing text in the forum editor? Didn't had that before...

Slow Typing Thread

Which also links to another slow typing thread.

The syndrome began right after the creation of the XMega sub-forum. :roll:

JC