Question about serial port modes \ VOGONS

Question about serial port modes

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First post, by videogamer555

Posted on 2017-01-12, 20:54

videogamer555 Offline

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What's the difference between the Modem and Null Modem modes in DOSBox? With real hardware, a null modem cable is one where the DataTX pin on each end is connected to the DataRX pin on the opposite end (which differs from a normal serial cable where the TX on one end is connected to TX on the other, and RX on one end is connected to RX on the other). However, with DOSBox, there are no physical pins to connect. So how does the Null Modem mode differ from the Modem mode?

And on a somewhat related topic, how do you use the non-data serial pins (CD, RI, DTR, DSR, CTS, RTS) when emulating serial communications in DOSBox (using modes Null Modem or Modem, rather than using the directserial mode)? Or does DOSBox simply not emulate these pins?
CD = Carrier Detect
RI = Ring Indicator
DTR = Data Terminal Ready
DSR = Data Set Ready
CTS = Clear To Send
RTS = Ready To Send

Reply 1 of 3, by Jorpho

Posted on 2017-01-12, 21:32

Jorpho Offline

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If I'm not mistaken, the "modem" mode emulates an actual dial-up modem, with Hayes commands and so on.
DosBox Guide to Multiplayer Games

videogamer555 wrote:
Or does DOSBox simply not emulate these pins?

http://www.dosbox.com/DOSBoxManual.html#Multiplayer suggests that DOSBox supports RTS/DTR handshaking.

Reply 2 of 3, by videogamer555

Posted on 2017-01-13, 04:59

videogamer555 Offline

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Jorpho wrote:
If I'm not mistaken, the "modem" mode emulates an actual dial-up modem, with Hayes commands and so on. DosBox Guide to Multiplay […]
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If I'm not mistaken, the "modem" mode emulates an actual dial-up modem, with Hayes commands and so on.
DosBox Guide to Multiplayer Games

videogamer555 wrote:
Or does DOSBox simply not emulate these pins?

http://www.dosbox.com/DOSBoxManual.html#Multiplayer suggests that DOSBox supports RTS/DTR handshaking.

Very interesting. That should be useful for my next project.

Just one thing though. If I'm not mistaken, doesn't RTS go to CTS (not DTR), and doesn't DTR go to DSR? RTS and DTR are both output pins on a serial port, while CTS and DSR are both input pins on a serial port.

Reply 3 of 3, by videogamer555

Posted on 2017-01-13, 23:20

videogamer555 Offline

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Interesting how dosbox simulates serial port control signals over TCP/IP. It sends a 0xFF byte (acts as an "escape" character) followed by a byte with bit flags indicating the states of the serial port's 2 control output pins (RTS and DTR). If it actually has to send a 0xFF byte in the serial data streem, it sends a 0xFF byte followed by another 0xFF byte. All data bytes it sends other than the 0xFF data byte, are sent without the additional 0xFF "escape" byte preceding the data byte.

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