Serial and analog connections for ZX-24x

[wwwoholic]

After long time of using ZX-24n and ZX-1280n my customer is looking for compromise solution with small size and multiple serial channels (software-based won't do). I suggested switching to Xmega but there are several concerns due to different voltage.

First, we used 74LS04 before to connect hardware ports to external devices with TTL-compatible RS-232 inputs. As far as I know 74LS04 does not work at 3.3V and I did not find recommended schematics for 32A4 devices. On other thread you recommended 74LVC3G04 alternative and it is OK for 3.3V, but it only has 3 invertors. Is there anything suitable with more?

Second, I wonder how versatile is the scematics based on logic invertors. At 5V we were able to reliable communicate with both TTL-compatible and with true RS-232 inputs (like PC com ports). I guess you are confident that it works since you are using this for programming port. Were there any programming/debugging issues with 24x devices?

Third, we need multiple (up to 8 ) analog inputs, usually a mix of Hall-sensors and resistors. When resistors were used they had independent 5V supply. Now we have to use 3.3V output from ZX and since L78L33ABUTR is only rated at 100mA I am not sure it would be enough to use as supply for sensors.

[dkinzer]

I did not find recommended schematics for 32A4 devices.

They are on the Downloads page: http://www.zbasic.net/doc/ZX-32a4_Schematic.pdf

On other thread you recommended 74LVC3G04 alternative and it is OK for 3.3V, but it only has 3 invertors. Is there anything suitable with more?

The 74AHC series can operate at 2.0V to 5.5V while the 74HC series can operate at 2.0V to 6.0V. Search for the 74AHC04 and 74HC04.

Were there any programming/debugging issues with 24x devices?

We have not seen any issues with the inverter-based level converters. That said, a "0 to positive" voltage swing does not meet the RS-232 specifications so it is entirely possible that you could encounter an RS-232 receiver that does not recognize the logic zero output of an inverter as a "mark" signal.

Now we have to use 3.3V output from ZX and since L78L33ABUTR is only rated at 100mA I am not sure it would be enough to use as supply for sensors.

3.3V regulators are fairly inexpensive. If you get one in a TO-220 package and mount it with a heatsink you should be in good shape. We use the MIC2937A-3.3WT on the ZX-128A1 Development board.

[wwwoholic]

Hi Don, thanks for the prompt reply.

On the schematics page the circuit for MAX3232 has Reset pin connected to the second line driver input of the MAX. What is that for? Just to not leave it floating?

[dkinzer]

On the schematics page the circuit for MAX3232 has Reset pin connected to the second line driver input of the MAX. What is that for?

That connection shouldn't be there. It may have been left over from an earlier rendition of the circuit.

[dkinzer]

That connection shouldn't be there.

The document has been updated.

[wwwoholic]

Thank you.

I have one more question regarding ZX-24x schematic itself. It shows B.1 grounded. Why did not you multiplex it with something else like you did with six other pins? It functions as ADC9, which might be useful for applications requiring many analog inputs.

[dkinzer]

Why did not you multiplex it with something else like you did with six other pins?

It was mostly a routing issue. The other pins to which it could have been most easily connected were already doubly connected. Further, having an ADC input grounded provides a means for run-time calibration of ADC offset. That is, if you perform a conversion on a grounded pin you should get zero. If the result is non-zero, that is the offset to apply to other readings.