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ComLynx<->USB Adapter


42bs

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Hi, since RS232 is mostly gone in modern PCs, I wanted to use one of the many USB<->TTL adapters.

Looking around there are many with a Prolific chip, some with CP2022 and also some with FTDI.

 

Prolific is bad, as these allow only the "standard" baudrates. But CP2022 accepts also the Lynx-Baudrate of 62500Bd!

Important: Most TTY examples check only for those standard baudrates. But Windows sends _any_ down to the driver.

Reading back returns the actual baudrate used!

 

Important is, that those chips are 5V tolerant, so there is no level-shifter needed!

 

The straight forward connection GND to GND ComLynx RX/TX to RX/TX of the CP2022 works for sending from PC to Lynx, but not from Lynx to PC. One needs a small (Schottky) diode from ComLynx to TX (see gfx).

 

post-3110-0-37194700-1520685306.png

 

I used just a small SMD diode I found on some old PCB. With this, data transfer with 62500Bd works perfect.

 

Cheers

Edited by 42bs
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  • 5 months later...

I found a already equipped board with a dual interface using this chip:

https://www.silabs.com/documents/public/data-sheets/CP2105.pdf

 

Nice thing, you can connect two lynx at one USB cable. but only one of the connections features "unusual" speeds like 31250 and 62500.

 

Annoying thing, shipping is more expensive that the boards :-(

 

42bs: I was wondering about the diode. its clear that it prevents the TX from pulling the line while the lynx is sending. but wouldnt a resitor do a better job?

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I found a already equipped board with a dual interface using this chip:

https://www.silabs.com/documents/public/data-sheets/CP2105.pdf

 

Nice thing, you can connect two lynx at one USB cable. but only one of the connections features "unusual" speeds like 31250 and 62500.

 

Annoying thing, shipping is more expensive that the boards :-(

 

42bs: I was wondering about the diode. its clear that it prevents the TX from pulling the line while the lynx is sending. but wouldnt a resitor do a better job?

 

You mean two Lynx'es w/o ComLynx-ing? While developing T-Tris, I did download the game to up to 4 Lynx in paralell w/o problem.

 

Regarding the diode: I found the hint in another forum. I tried an resistor but it did not work. Too bad, I cannot remenber the explaination ...

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the inactive state is 5V. When some other lynx sends a 0 we don't want our Lynx 5V to block the communication. A diode works as an AND gate.

 

Just to check if I understand: The inactive state of the TX port is 5V. If connected directly with the RX/TX line, the Lynx cannot sink enough current to pull the line below the 0-level (IIRC < 1.5V).

So the diode will prevent current from the TX to the RX/TX line. So a sufficiently high resistor might also work.

Am I right?

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You mean two Lynx'es w/o ComLynx-ing? While developing T-Tris, I did download the game to up to 4 Lynx in paralell w/o problem.

 

Regarding the diode: I found the hint in another forum. I tried an resistor but it did not work. Too bad, I cannot remenber the explaination ...

Not so much for the uploading.

 

PS: parallel upload to several lynxes never worked reliable for me.

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ComLynx is open collector circuit. You can connect many Lynxes in parallell. If you use resistors then every Lynx will source some current. The talking Lynx may not be able to sink the amount. Using a diode blocks the current flowing from inactive 5V to active 0.

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sage, the difference is that the CP2022 solution is powered by the USB. The MAX232 solution is powered by the Lynx and I doubt that the 5V does provide 500mA as the USB can.

The Lynx simply can't pull down the 5V from the CP2022.

It wasn't like it did not work at all but the received values were corrupted.

Edited by 42bs
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