I am working on a remote sensor that will communicate digital information via a serial interface. I am looking for some ideas in creating such an interface that has some significant design constraints. The sensor and the host both use PICs, and communication will occur between their respective USARTs. I am hoping that some "free-association" between like-minded (and even different-minded!) engineers will help to lead me to an acceptable solution.
The data rate will be less than 2 kBaud (RS-232 data format) over a 6 to 8 foot wire (bidirectional), which is, in most cases, very easy! But here are the constraints that make it challenging:
1. It must be cheap, cheap, cheap! I don't want to incorporate any (integrated) line drivers into the solution.
2. Communications should be relatively reliable (I can do error detection and correction) in high noise environments, but not necessarily perfect.
3. I don't mind using twisted pair, but I don't want to use shielded cable. Therefore, signal swings should be kept to a minimum. I like LVDS, but the transceivers are prohibitively expensive, use lots of power, and are overkill for my proposed data rates. I've also considered current modulation, as opposed to voltage modulation, too keep the radiated noise down.
4. Immunity to 4KV human body model static discharge is a must on both host and sensor. Additionally, it must meet CE for radiated RF emissions.
5. Ultra-low power! I'd like the entire comm solution (both ends) to consume less than a few hundred microamps at full speed. Please don't propose RS-232 line drivers...too much power (and too expensive).
6. The cable will also provide power (9V from an alkaline battery) from the host to the sensor.
I know that I can simply wire the ports together over the line, but this will provide no ESD protection and signal reflection due to mismatched impedance may cause data errors. Also the large signal swings and sharp edges can cause RF emission problems, especially in an unshielded cable.
FYI, both microchips will be operating at either 3 or 3.3V.
Any ideas?
The data rate will be less than 2 kBaud (RS-232 data format) over a 6 to 8 foot wire (bidirectional), which is, in most cases, very easy! But here are the constraints that make it challenging:
1. It must be cheap, cheap, cheap! I don't want to incorporate any (integrated) line drivers into the solution.
2. Communications should be relatively reliable (I can do error detection and correction) in high noise environments, but not necessarily perfect.
3. I don't mind using twisted pair, but I don't want to use shielded cable. Therefore, signal swings should be kept to a minimum. I like LVDS, but the transceivers are prohibitively expensive, use lots of power, and are overkill for my proposed data rates. I've also considered current modulation, as opposed to voltage modulation, too keep the radiated noise down.
4. Immunity to 4KV human body model static discharge is a must on both host and sensor. Additionally, it must meet CE for radiated RF emissions.
5. Ultra-low power! I'd like the entire comm solution (both ends) to consume less than a few hundred microamps at full speed. Please don't propose RS-232 line drivers...too much power (and too expensive).
6. The cable will also provide power (9V from an alkaline battery) from the host to the sensor.
I know that I can simply wire the ports together over the line, but this will provide no ESD protection and signal reflection due to mismatched impedance may cause data errors. Also the large signal swings and sharp edges can cause RF emission problems, especially in an unshielded cable.
FYI, both microchips will be operating at either 3 or 3.3V.
Any ideas?