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Fez Mini RS232 COM1 With Serial to USB Cable Problems


Hi, I am using COM1 on the Fez Mini to communicate with a PC. I have discovered that, unlike any other device I have used, the com port will receive, but not send using a serial to USB converter cable. In this case, the cable is based on a Prolific PL-2303 chip. The com port sends data correctly whilst strictly using a serial cable, but not with the serial to USB cable. Any suggestions? Perhaps a particular brand of cable this might work with? The PC I am trying to connect to does not have a dedicated DB9 serial port, and it will not be easy to add one.


I am assuming you have a cable with RS232 circuit built in it? The RS232 interface on Mini is simplified and it relies on having the right signals on your side to work. Note that you must have both TX and RX signals connected even if you only need one of them


On the mini side both the TX, RX and GND are connected to a DB9 plug. That DB9 is connected to another DB9 which houses the RS232-USB hardware and connects to the PC via a USB port. Like I said, I can receive, but not send. If I do not use the RS232-USB cable, and use a straight DB9 cable, I can do both. Any ideas?


You can’t send from FEZ Mini or from the PC?


With a regular DB9 Serial Cable:
I can send data from the PC to the Fez Mini AND from the Fez Mini to the PC.

With a DB9 to USB cable:
I can send data from the PC to the Fez Mini, BUT NOT from the Fez Mini to the PC.


Ok, I think the problem is that FEZ mini simplified RS232 circuit relies on the RX line negative voltage to produce TX. I am near positive your cable is not providing RX232 signals as it should. Do you have a another USB-serial cable of different type?


I will have to buy one to try. I am also buying a low profile RS232 DB9 PCI card in case.


@ Gus: the Prolific PL-2303 chip is one of the most used chips in USB->serial cables so there should be no problem using such a cable


I know, GHI support those. The problem is not on the prolific side but the RS232 level converter inside the cable


I ordered and installed a FTDI based adapter, and this one works fine.