Oric V23 Modem

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Dbug
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Re: Oric V23 Modem

Post by Dbug »

Looks like the hint maybe that this interface only uses the ACIA 6551 without anything else behind.

All the RS232 interfaces I've seen that are based on the 6551 seem to use a MAX 232 as well, to do something the signals.
http://oric.free.fr/hardware.html
- you don't need the MAX232 circuit if you only want a TTL interface (e.g:
connecting to a minitel)
- no +12/-12 needed, the MAX232 chip provides EIA compatible levels (9V)
- as the MAX232 chip only allows two inputs and two outputs, I had to choose
one input among DCD, DSR and CTS; and one output among DTR and RTS.
DCD seems to be the most important input if you connect a modem to the
interface (it tells you when you are connected), that's why I choose it.
Also, I have chosen DTR rather than RTS in order to tell the attached
equipment when the cpu is ready, and this signal is usually connected to both
DSR and DCD in a null-modem cable.
http://retro.hansotten.nl/6502-sbc/lee- ... acia-6551/
The MAX232 performs level translation between the TTL levels of the 6551 and the bipolar RS232 levels. Only the signals for a 5 wire interface are implemented with DCD and DSR being tied low on the 6551 to allow continuous operation.
So possibly this Oric RS232 interface is only usable with the provided V23 modem?

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Re: Oric V23 Modem

Post by Chema »

iss wrote:
Fri Apr 30, 2021 6:59 pm
Dbug wrote:
Fri Apr 30, 2021 6:28 pm
Should I just get this +5 signal from somewhere to CTS and or DSR as suggested by Chema?
I really don't understand this idea ?! If this is to permanently activate (for instance) CTS - you should feed minus 5V relative to GND and disconnect the other side which sends RTS
Could be nonsense, really. I always get confused about when 5v is used as active, instead of -5v. I recall that this was one of these occasions. The idea is force an unconnected signal to active, so the other part does send the data. That is why I suggested trying with those unconnected pins which are involved in the hardware flow control.

I just found this in this webpage:
What to do when the Instrument needs DTR Low and RTS high: Removing the Handshaking
Some software, like the Windmill COMIML serial driver, use the DTR line for handshaking (flow control), setting it high to indicate that it is ready to receive data. Some instruments, though, need DTR to be low and RTS to be high before they will provide data.

In this situation you need to change the connections on the RS232 cable.

Connect pins 8 and 7 (i.e. CTS drives RTS)
Connect pins 1, 4, 6 This should maintain the DTR line in the correct state, by connecting it to DCD and DSR

This would normally be done at the instrument end rather than the PC end of the cable.

Also check that transmit at one end goes to receive at the other.
In any case, with the null-modem cable Dbug is getting data, but missing one byte every two... that is indeed strange. If he gets correct data in every other byte, speed must be ok... parity is also most probably ok, or the wrong data would be random (depending on the 7th bit value). Flow control should be working unless wired up in a weird way...

Some hardware strips down automatically bit 7, but not sure if this is the case here. If the number of stop bits is too low and the Oric does not have time to process one byte when the next comes, you could have this kind of behaviour? Not sure. This could be tested by reducing the transmission speed.

But the fact that some characters arrive correctly is good news...

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Re: Oric V23 Modem

Post by Chema »

By the way, an oscilloscope or signal analyser would do wonders debugging this... You can check the actual signal values when sending/receiving a byte and see what is going wrong whenever you get receiving errors.

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Re: Oric V23 Modem

Post by Silicebit. »

Hi Mike! I think you don’t have a real RS232 interface. Please, could you take a pic of inside of interface?
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Re: Oric V23 Modem

Post by Dbug »

Silicebit. wrote:
Tue May 04, 2021 7:34 pm
Hi Mike! I think you don’t have a real RS232 interface. Please, could you take a pic of inside of interface?
Not possible, it's all in a blob of melted plastic, but the author of Oricomms did it back in the days, the schematics look like that:


That was all explained in a series of OUM articles:

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Re: Oric V23 Modem

Post by Silicebit. »

Ok, here is the schematic.
Oric serial.
Oric serial.
This interface is a serial TTL interface, not RS232 compliant. You need a MAX232 IC to translate TTL levels to RS232 bipolar levels and viceversa.

Please see TIA/EIA-232-F standard https://cw.fel.cvut.cz/b191/_media/cour ... busses.pdf

And MAX232 datasheet https://www.ti.com/lit/ds/symlink/max232.pdf

¡¡Please, STOP connecting the interface to RS232 port on your PC. Your PC is sending +12V, -12V pulses to a TTL input (+5V, 0V)!! Maybe the 7404 or the 6551 ACIA are already fried. :-(
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Re: Oric V23 Modem

Post by Chema »

Silicebit. wrote:
Tue May 04, 2021 9:25 pm
¡¡Please, STOP connecting the interface to RS232 port on your PC. Your PC is sending +12V, -12V pulses to a TTL input (+5V, 0V)!! Maybe the 7404 or the 6551 ACIA are already fried. :-(
Quite a sensible advice! That is why I asked some posts ago about the voltages. Checking this is important. However, I doubt that any USB to RS232 today uses anything else that +-5volts (not sure if some may use 0-5, which is outside the norm, but not uncommon), but certainly there were some that pulled up the voltage to +-10v some years ago.

What really puzzles me is that he is sending everything correctly and receiving one byte every two. I am really excited to see where this thread gets to :)

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Re: Oric V23 Modem

Post by Dbug »

Yeah, the usb adapter I use definitely don't send anything above 5 volt.

Regarding the various "232" things available, what about these FTDI "TTL-232R-5V" or "TTL-232R-3V3", is that equivalent to adding a MAX232, or is it just another USB to serial?

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Re: Oric V23 Modem

Post by kenneth »

With a DB9 connector on the old PC you have to add a MAX232 chip to make the signals of the Oric interface compatible. (The Max is a "pump" that creates +12v -12v from a 5v power supply). With the "5v" USB cord this is not necessary, the levels of the signals are 0-5v.

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Re: Oric V23 Modem

Post by kenneth »

The Oric interface uses 5v signals, which makes it compatible with the Minitel device which is why there is no Max242 chip to convert the voltage signals, even if the inputs of the minitel tolerate voltages up to 18v and -18v.

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Re: Oric V23 Modem

Post by Dbug »

kenneth wrote:
Thu May 06, 2021 12:50 pm
The Oric interface uses 5v signals, which makes it compatible with the Minitel device which is why there is no Max242 chip to convert the voltage signals, even if the inputs of the minitel tolerate voltages up to 18v and -18v.
So, based on the schematics, what would be the simplest way to get this working as a proper RS232 compatible with a PC?

Since there's no VCC accessible on the outside on the interface to power a chip, I guess it needs to come from the PC side?

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Re: Oric V23 Modem

Post by kenneth »

I saw the technical data of the usb wire TTL-232R-5V, normally we can connect it directly on the Oric interface the level of the voltage is 5v only. The french Minitel terminal uses this same level voltage. The problem with 0-5v level, the wire to communicate is shorter than the -12, +12v level mode.

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