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Missing the mark
Have you ever gotten a product and thought, “Boy the designer just missed the mark?” So close, but then just stopped before getting to the finish line. I recently had that experience when buying Future Technology Device International’s USB-COM485-PLUS1. This has the potential to be a great device. I was working on a project where my board had to talk RS485. To make sure I got it right I needed something that would convert 485 into a virtual COM port so I could view what I was transmitting from my board.
How it works
The RS485 end has a DB9 connector. They use the SP481EC ($1.77) to convert a half-duplex RS485 signal to a standard UART output. This output is the same output you would get out of an MCU. The UART style output is then converted to USB using a USB to serial chip FT232RQ ($4.50). The now USB signal exits the board on a USB-B connector.
The DB9 has a 120-ohm resistor connected to Data- that the user can connect to Data+ as a termination resistor if they choose. There are also 3 LED’s on board. One tells you the USB is connected. One flashes when a transmission is received the other when one is sent. It’s a pretty nice setup. It does not come with a protective shell but has a drawing in the datasheet if you would like to make one yourself.
Why is it only have a design?
For an additional $1.20 they could have gone full duplex. It never even occurred to me that anyone would deliberately design a converter as half-duplex, so I was unpleasantly surprised when I started to hook it up to my full duplex board. If the target board is half-duplex, then a full duplex converter can be configured to talk with it. However, if you have a full duplex design, like I did, and a half duplex converter you are SOL.
After looking at their product a few days I decided to fix their design. It required removing just one chip, the SP481EC. I replaced it with the SP3496EEN-L ($2.97).
Figure 1. Hacked half-duplex FTDI RS485 to virtual COM port.
Sometimes you get lucky and can drop the new chip on the existing footprint and just move a few wires. This day I was unlucky and had to dead-bug the chip and run 8 wires. I used 2 more of the DB9’s unused pins. Even so there are two more unused lines were another termination resistor could be placed for the second half of the communication channel. So there really is no downside to going Full-Duplex.
Some questions
This is a very useful product. It worked the first time I plugged it in. They added some very nice indicator LED to let me know things were working. They designed it so that it would fit into a nice box. So why not sell the box with it? Or at least offer it for sale as an add-on? And why go half duplex instead of full duplex. The cost difference was $1.20 on a $26 product (a 4.4% discount). A half-duplex design could use either one but a full-duplex design must have a full-duplex converter.
I don’t know if this was just short sightedness on the part of the design team or a bad decision was made by a bean-counter. Either way I think they missed a great opportunity to make a better product.
Big deal!
So why spend an entire blog complaining about this. First there was no alternative that I could find available to buy at a comparable price. Second, I have never done a product review before and thought it would be fun. Third, I wondered how often I have designed something and thought it was just the best thing since sliced bread but someone else thought, “Wow, so close.” I think it is a good idea to reflect on both our own and others designs so that we can make better designs and better decisions in the future.
Final thoughts
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