TTL inputs on MD888+

[Andy]

Hello Tri team

We are planning to use your PLC's for our EnviromentControllers. We do Temperature and Humidity measurement using Sensirions SHT75 Sensors with TTL Output. As far as we have seen, this is only possible using the 4040 Expansion Board (TTL inputs). Can we use it also together with the HMI-MD? Or can we use the Digital I/O's therefore? Please advise, thanks

[support]

You mentioned "temperature" and "humidity" sensors. Wouldn't these be analog type of output? "TTL" refers to 5V digital logic voltage where > 2.0V represent logic 1 and < 0.8V represent logic 0.

If you can confirm that your sensors is analog voltage of 0-5V you can connect them to the PLC's analog input. But if these are really TTL level (maybe a pulse trains whose frequency represents the magnitude of the measurand) then you can convert the TTL voltage level to the PLC's NPN input level using a low cost ULN2003A or ULN2803A IC. The circuit can be download from the following link:

http://www.tri-plc.com/appnotes/TTLtoNPN.PDF

[Maxi]

Thank You for the fast answer.
Yes, it's TTL Output from the Sensor (SHT75).
They also use 8-bit CRC which i read somewhere is not good to handle with the PLC?. We also looked at the Dallas 1820 1-Wire Temperature sensor/transmitter but they also use crc causing more complex calculations.
Using the ULN2803A IC and NPN Input sounds very symphatic to us. Have you a sample how to capture/process this style of input (like sample for Analog I/O in the Manual)?
We have already seen possibility to use 4-20mA with 250 Resistor to get 1-5V. This would be used for some Sensors. Unfortunately, Temperature and Humdidity is not covered without expensive transmitters. We have found something interesting @ Airtesttechnologies.com, supplying 4-20mA for Temp, RH and CO2. We contact them.
Thanks for the fast and accurate answer, very good service!

[Andy]

Hello Tri Plc Team

Are u there? Seems that nobody reads my second question, or am i asking something wrong?

Your Suggestions are very needed!

[support]

Sorry, your response didn't sound like a question to us. Anyway, if your sensor has a communication protocol, then the simplest type to interface with our PLC will be one that support MODBUS RTU or MODBUS ASCII protocol. The PLC has built-in support for these protocols so it is very easy to use. If yous sensor can communicate using CR-terminated ASCII strings then you can use the PLC's built-in PRINT #, INPUT$ or NETCMD$ to communicate with your sensor. For hardcore type communication device, the TBASIC support 16 bit CRC computation but you have to write the whole driver using INCOMM and OUTCOMM to talk to the sensor.

We don't support Dallas 1-wire protocol.

[Andy]

Thank You for the Answers, no problem about.

As i understood correct, we have to write the Driver if we like to use Dallas or the Sensirion devices. All of them have some output sequences. As we understood, it shouldn't take to much work to write the driver in TBasic.
We'll try it.

Again Thanks for the answers