November 30, 2020

An Isolated USB Null Modem Connection!

2 min read
Serial connections are still very prevalent today, especially in hobby electronics. In most cases, a...

Serial connections are still very prevalent today, especially in hobby electronics. In most cases, a direct connection between two systems is quite simple, but there are many gotchas that can cause issues and even damage to the connected equipment. Because many laptops and other electronics can have a trickle of current from the AC source (because of the class Y capacitor across the isolating transformer), if the two connected devices have different ground potentials, current can flow through this capacitor (or, in non-isolated power supplies, directly) and can easily cause damage to the connected devices.

I ran into this issue with an FTDI breakout board once — I connected it to a device on a different circuit from my laptop, and suddenly there was a puff of smoke and the ground track on the PCB vapourized. This luckily cleared the fault, but in certain cases, you could end up with substantial damage. Amazingly, after I repaired that trace, the breakout board still worked, but the whole situation could have been avoided. This is the kind of situation where you don’t know you need it until you need it!

If you happen to own an isolation transformer, that’s great — but these are bulky and can be expensive. However, you can pick up this fully isolated USB-to-serial-to-USB device from Geppetto Electronics! The isolation is provided by four Toshiba TLP2270 optical isolation ICs. These are capable of speeds up to 20 Mbaud, as well as differential voltages of up to 5000 Vrms!

Instead of the ubiquitous FTDI FT232 USB to serial chips, Geppetto has instead used the CY7C65213A from Cypress (now Infineon); chosen, in part, because the Cypress chip uses a generic driver and is therefore safe from any “FTDI-gate” type of issue. These have an almost identical feature set to the FTDI chips, but are about $2 less expensive in single quantity. If you wanted isolated USB to TTL serial, you could tap into the signal between the isolators and the Cypress chip quite easily.

A great choice for a low-cost, well-designed board like this! Pick one up and help protect your serial comms from ground potential differences.

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