Part of my real job is traffic counts, and I’ve been working on some permanent traffic counter setups.  The counters that we are using have an RS-232 interface, although in certain situations, an RS-485 connection must be used.

They don’t make computers with RS-485 connectors on them (anymore).  So I’ve purchased an adapter, and then a coworker purchased another (more expensive) adapter.  The differences are interesting.

Adapter #1

Adapter #1 is an inexpensive $10 on Amazon (affiliate link). I’m not going to post an image of the inside, but there are 4 chips:

1. an ME7660C Charge Pump (converts +5V to -5V)
2. A 4093 Schmidt Trigger
3. Two SP485EE RS-485 Interface chips

The reason for two RS-485 Interface ICs is because RS-485 is full-duplex, so my guess (without tracing the circuit board) is that one chip handles the positive RX/TX and the other handles the negative RX/TX.

Adapter #2

Adapter #2 is a far more expensive converter (it’s $80 on Amazon, affiliate link). Similar to above, I’m not going to post an image of the inside, but it also has 4 chips… some similar, some not so much:

1. 7660C Charge Pump
2. MC14093 Schmidt Trigger
3. SP3220 RS-232 Transciever
4. SP485EEW Full Duplex RS-485 Interface

This particular adapter was recommended by the manufacturer of the traffic counters we are using, and my thought is that it has to do with the fact that it has a full-duplex interface.  I looked on Digikey to see if they have both types of chips, and they only stock the half duplex chip (for less than a dollar per chip).

I was so far not able to get either of these to work with the traffic counter, although I’m willing to bet that in both cases, it has to do with something that isn’t the device (my wiring or needing to use a null modem adapter).

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