Max baud rate for SN75176B

Hi,

Has anyone used the SN75176B RS-485 transceiver at 1MHz (1Mbps) and above?
I only need to go about 10 feet (3 meters).

Would the CAT5E cable be OK for that?

Thanks.

Thae data sheets suggest it would be unlikely to work above about 30MHz so yes, at 1MHz it should be OK. Be careful when you specify 1M bits per second and 1MHz though, they are not quite the same thing. To send bits you probably mean 1s and 0s and the rate of change from one to the other is more important than the frequency they occur.

I've used CAT-5 at much higher frequencies that 1Mb/s, provided you are careful to maintain the balance between wires it should be OK to beyond 100Mb/s over short distances.

Brian.

Thanks, Brian.

I really meant 1M bits per second. I should have never said 1MHz.

So, what do you estimate to be the top Mbps you can achieve with a SN75176B over a short distance? Is it 30 Mbps or is it less?

It's difficult to be specific because there are several factors to consider when pushing speed to the limit.
The actual calculation is simple, add the output rise time and fall time together, this will tell you how fast the output can theoretically change but the figure will assume there is no bit length, just a rising slope and falling slope, therefore a triangular shaped signal. At the receiving end you have to consider the amplitude of the signal, this drops as the speed increases and length of cable increases. Generally as long as the differential input threshold is reached the output of the receiver will change and you will get a logic level change at the output. The speed restriction will primarily be due to the shape of the signal degrading to the point where the logic output no longer gives an accurate enough copy of what went in at the transmitter. As the speed increases the edges of the signal will be less well defined, it's really up to the rest of your circuit to judge if the signal can be interpreted.

As a guess I would say 30Mbs is pushing your luck, somwhere between 10 and 15Mbs is a more realistic ceiling. You might be lucky though...

Brian.

When working with high speeds don't forget about terminations (100-120ohm) and bias ..
Specially termination resistors are very important ..
see:
Termination and Bias Resistor Information - National Instruments

:wink:
IanP

I fully agree with @IanP, it's worth adding that the resistors are needed at both ends of the cable, even on a short run and they should be positioned as close to the ends of the cable as possible, especially as you are pushing for highest speeds. Stick to 100 or 120 Ohms, they must terminate the CAT-5 at it's characteristic imedance or you will get unwanted signal reflections from the cable ends. The 'Bias' resistors to ground and +Ve are not essential if you are sure there will always be an active driver on the cable, they are there to establish a default voltage if the driver is in a disabled state.

Brian.