If anyone can think of why this idea may NOT work, please let me know - I want to try to build this. I need to make a serial signal breakout box, and I can't afford the card... so I want to take RS-422/485 signals, convert them to TTL with a converter chip, and use a CPLD as a Mux for the several TTL signals connected to it. The TTL signal that passes based on Mux setting is converted to RS-232 with a MAX232 chip, and goes to the PC. Can a XC9500 CPLD be made fast enough to keep up, for baud rate of 115kbs?
If anyone can think of why this idea may NOT work, please let me know - I want to try to build this. I need to make a serial signal breakout box, and I can't afford the card... so I want to take RS-422/485 signals, convert them to TTL with a converter chip, and use a CPLD as a Mux for the several TTL signals connected to it. The TTL signal that passes based on Mux setting is converted to RS-232 with a MAX232 chip, and goes to the PC. Can a XC9500 CPLD be made fast enough to keep up, for baud rate of 115kbs?
A UART in the PC takes (if I remember correctly) 16 samples per bit. I guess you should be safe if you sample at least at 16x the 115K2 bit time (equals >1.8Mhz)... you should be able to get this out of your CPLD...
It sounds to me like you don't need any programmable logic at all. Why can't you just use however many you need of the two kinds of converters (RS-232TTL and RS-422TTL) and use an analog or bidirectional mux or something on the logic level signals connecting them?
If there is some reason why you want to use the programmable part, for example if the selection logic is more complicated than I am imagining, then I don't think you will run into any speed problems. I do not consider
115kbaud to be fast for a digital circuit.
RS-232 and RS-422 are largely compatible, in that each device has an input and an output signal.
However, there ARE some issues surrounding interfacing of RS-485 with RS-232.
RS-485 is a shared bus architecture, meaning that the same pair of data lines is used for transmitting and receiving. This necessitates some kind of scheme to disable your drivers when they are not driving. Some converters (see B and B electronics) sense transitions on the RS-232 line, and automatically start driving data onto the RS-485 line for a certain amount of time after the last detected transition. Then they go back to tri-state mode. This additional logic would complicate your circuit quite a bit.
Let me know if you want me to elaborate on any of these points.
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