Is a FTDI USB to RS232 better than a real RS232 COM port? - windows

Question

I am using a Zebra DS457 Scanner to read bar and qr codes via COM-Port (RS232). In my test evironment I used a MSI terminal with Win10 and it worked on the real COM-Port without any problems. But on other devices (Win10 and Win7) there are some issues that the software trigger does not come through and the read information do not get sent back to the computer. When I am using a USB to RS232 FTDI adapter I have no issues at all. But why? First I thought it is Win10 and the legacy support could be better, but the adapter is on all devices better and faster. How is this possible? Maybe a driver specific thing? I am using this adapter link to conrad.de.

Answer 1

An FTDI serial port will impose a minimum latency between the time a character arrives over the wire and when an application can see it, and between the time an application wants to send something and the time it goes over a wire. On older devices, these latencies were a minimum of 1ms each, but I think some newer high speed devices have reduced them to 125us. Further, data which arrives at just the wrong speed sometimes ends up with hundreds of milliseconds of additional latency, for reasons I don't quite understand.
On the other hand, an FTDI device can buffer 256 bytes of data from the wire, or 128 bytes of data from the USB port to be sent over the wire, and process RTS/CTS handshaking, without any software intervention--abilities which are lacking in the UART chips used by PC serial ports. If software gives 128 bytes to an FTDI device, it will start sending it until the remote device deasserts its handshake line, whereupon the FTDI device will stop sending as soon as the current byte is complete; it will then resume transmission as soon as the remote device reasserts handshake. If an FTDI device receives enough data over the wire that its UART would be in danger of overflowing, it will automatically deassert its handshake output without requiring any software intervention. The UART used in PC serial port, by contrast, requires a fast interrupt handler to control or respond to the handshake wires. If an interrupt handler maintains a 4096-byte buffer, it may deassert the handshake wire once that buffer is 75% full, but nothing would deassert the handshake wire if the buffer is less than 75% full and 17 bytes arrive over the wire in quick succession before the UART interrupt handler. Worse, if transmit buffering is enabled, and the PC has fed 16 bytes to the UART for transmission when the remote device deasserts its handshake line, those 16 bytes will be sent out whether or not the remote device is ready to receive them (and based upon the handshake wire, it very well might not be).
Thus, some applications can work much better with an FTDI UART, and some much better with an actual serial port.