Q68 serial port

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FrancoisLanciault
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Q68 serial port

Postby FrancoisLanciault » Tue Dec 26, 2017 6:03 pm

Hi,

time has come to connect my Q68 to my SGC QL in order to transfer files.

Some stupid questions:

The manual says that "the original (unreliable) serial ports are not supported". Is the Q68 serial device name still ser1 / ser or some new name ?

Also as there are no handshaking possible, it means I only have to connect the Tx, Rx and Ground ? I guess ground connection is needed for reference to the High and Low signal voltage ?

Finally, just to be 100% sure, is the serial port on the Q68 a real RS-232 (with -3/-12 and +3/-12 volts signals or a TTL serial (more common these days) with a 0 and 5 volts signal ? I assume it's a real rs-232 as some peoples have successfully transferred files from PC.

And if it is RS-232, how does the +/-12 volts generated from the 5 volts power input ?

I know, stupid questions, but you've been warned!

Thank you.

François


Derek_Stewart
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Re: Q68 serial port

Postby Derek_Stewart » Tue Dec 26, 2017 10:39 pm

Hi,

There is no 12v on the serial port.

The serial port device driver name is SER1

The handshake lines DTR, RTS are pulled high for use with the serial mouse.

The serial port pin out is not the same as the QL SER ports.

The QL SER ports are not very reliable. I only got 9600 Baud with SGC, SMSQ/E SERNET connection to QPC2 or Q60.

Can you not copy all your data into a QXL.WIN file and transfer the file to a SD Card via PC or QPC2.


Regards,

Derek
FrancoisLanciault
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Re: Q68 serial port

Postby FrancoisLanciault » Wed Dec 27, 2017 6:47 am

Thank you Derek for the information.

I know there is no +12V pin on the Q68 serial connector. But my question was about the signal themselves on the Rx and Tx lines. The RS-232 standard stipulate (from memory) that a bit value of zero is transmitted with a volage anywhere from -3 volts to -12 volts on the Tx and Rx line and a bit value of one is from +3 to +12 Volts. Or the other way around, as I said it's from memory.

Nowadays, what we see on newer devices (like the Arduino or the Raspberry Pi) is a TTL serial interface, also known as UART. The protocol is the same but the voltage level are different i.e. 0 Volt for bit value of zero and 5 Volts for bit value of one.

So my question, mainly, was just to confirm which one it is. I don't want to send 12 volts Tx signal into my new Q68 and realize it was a mistake when smoke is coming out of the board! From what I can understand it is a real RS-232, but as the Q68 is a brand new device it "could have been" TTL level serial lines. (Like for example on the ZXpand+)

The followup question, for Peter, was added only to teach me some basic electronic : If the voltage values for the serial lines are indeed something like +/- 12 Volts, how does one create those voltage from a 5 volts supply.

François


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janbredenbeek
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Re: Q68 serial port

Postby janbredenbeek » Wed Dec 27, 2017 12:14 pm

FrancoisLanciault wrote:Thank you Derek for the information.

I know there is no +12V pin on the Q68 serial connector. But my question was about the signal themselves on the Rx and Tx lines. The RS-232 standard stipulate (from memory) that a bit value of zero is transmitted with a volage anywhere from -3 volts to -12 volts on the Tx and Rx line and a bit value of one is from +3 to +12 Volts. Or the other way around, as I said it's from memory.

Nowadays, what we see on newer devices (like the Arduino or the Raspberry Pi) is a TTL serial interface, also known as UART. The protocol is the same but the voltage level are different i.e. 0 Volt for bit value of zero and 5 Volts for bit value of one.

So my question, mainly, was just to confirm which one it is. I don't want to send 12 volts Tx signal into my new Q68 and realize it was a mistake when smoke is coming out of the board! From what I can understand it is a real RS-232, but as the Q68 is a brand new device it "could have been" TTL level serial lines. (Like for example on the ZXpand+)

That's what I'd like to hear too before connecting my 1990's modem to the Q68...
The followup question, for Peter, was added only to teach me some basic electronic : If the voltage values for the serial lines are indeed something like +/- 12 Volts, how does one create those voltage from a 5 volts supply.

This can be done using a step-up converter ('charge pump') circuit. Such circuit was used in the Spectrum to generate +12 and -12V required for the RAM chips from the 9V power supply (actually the -12V was not used by the RAM chips but still laid out to the edge connector for use by the Interface 1 RS-232 port).
I doubt if the Q68 has such a circuit as it requires a coil to operate, and I did not see any sign of a coil on the Q68 board.
(I'm going to test it right now with a multimeter, I'll be right back in a moment...).

Jan.


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Pr0f
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Re: Q68 serial port

Postby Pr0f » Wed Dec 27, 2017 12:19 pm

For low currents you can do a charge pump with just capacitors and diodes, and a simple square wave generator. You can generate both + and - volts of higher than input voltage.

The MAX232 range of chips provide RS232 spec outputs from TTL signals and require only capacitors to achieve this. Linear Tech also have a similar range, so may be the RX and TX lines are buffered through one of these ? I haven't seen a circuit diagram for the Q68, so speaking from experience of other RS232 designs on single board computers...


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janbredenbeek
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Re: Q68 serial port

Postby janbredenbeek » Wed Dec 27, 2017 12:45 pm

Update: On the Q68's TxD line I measured about -5V when not doing anything ('Mark' level) so it's not quite at official RS-232 levels but still within specs.
I guess the RxD line will be compatible with +/- 12V too so it's probably safe to connect a modem.
It's still a pity that it doesn't support DTR/CTS handshaking as this is somewhat a requirement when connecting older equipment such as original QLs :(

Jan.


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Peter
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Re: Q68 serial port

Postby Peter » Wed Dec 27, 2017 9:51 pm

Yes the absence of hardware handshake on the Q68 is not ideal. I simply had no pins left.

But even with hardware handshake, the QL ports were not fully reliable at higher baudrates.

The RS-232 driver is a MAX3311E and contains a charge pump for the negative voltage, as you already guessed.


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Peter
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Re: Q68 serial port

Postby Peter » Thu Dec 28, 2017 10:11 am

FrancoisLanciault wrote:Also as there are no handshaking possible, it means I only have to connect the Tx, Rx and Ground ? I guess ground connection is needed for reference to the High and Low signal voltage ?

Yes for a data communication cable you need only Rx, Tx and the ground line as reference. For a serial mouse you also need some extra lines which provide power supply.


Derek_Stewart
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Re: Q68 serial port

Postby Derek_Stewart » Thu Dec 28, 2017 4:50 pm

Hi,

Would Software handshaking work, using XON/XOFF.


Regards,

Derek
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Peter
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Re: Q68 serial port

Postby Peter » Thu Dec 28, 2017 6:08 pm

The SMSQ/E implementation of the XON/XOFF protocol works up to 4800 baud from the QL, and up to 2400 baud to the QL.

For Q68 <-> Q68, or Q68 <-> PC, no handshake is required, because both sides are fast enough to handle 115200 Baud.



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