Warning: the pin female connector on the back of many computers is a parallel port not a serial port. While a null-modem serial cable will not attach to it as it also uses female connectors , certain other combinations of cables and gender changers can in theory be used to try to connect a parallel port on one computer with the serial port of another computer.
This should never be attempted, as it may damage one or both computers. For some applications, including terminal software when no hardware handshaking is used, very simple null-modem cables with three wires are sufficient to exchange data although not in the most reliable and efficient way.
This ensures, for example, that if there is an overflow in the serial port's input data buffer, the other side stops transmitting, and waits until the buffer is ready to receive more data. Amiga Explorer requires a complete null-modem cable, with 7 wires. High quality commercial cables follow this specification, but lower quality and 'do-it-yourself' cables sometimes differ.
If the wires are not connected properly, Amiga Explorer will not work. In some cases, including proper connections but poor quality or too long cables, Amiga Explorer may transfer short files, but it will fail with longer files in one or both directions. In other cases, the software will not work at all. The Amiga serial port also uses non-standard signals see the Amiga Forever documentation , which may damage the equipment on the other side.
This cannot happen with a properly wired null-modem cable. For example, pin 1 always connects to pin 1, and is always numbered '1' in all of the above illustrations and tables, regardless of whether it is 'male' or 'female', or viewed from 'inside' or 'outside'. All pins along this electrical path are numbered '1'. It is very important that the pins which on the different Amiga models supply power 9, 10, 14, 21, 23 be not connected to any active pins on the PC.
Also, the two different grounds shield ground and system ground should not be connected with each other. Innanzitutto cominciamo a fare una importante distinzione in base a cosa si vuol collegare, i computer e i microcontrollori sono chiamati DTE data terminal equipment in quanto considerati componenti terminali, mentre i modem e gli hub sono chiamati DCE data communications equipment perche' considerati un tramite per le comunicazioni, la differenza e' che per collegare due DTE i segnali RX e TX vanno incrociati realizzando il cosidetto cavo "null modem" mentre collegando un DTE ad un DCE i cavi vanno diritti in quanto l'incrocio e' gia' previsto all'interno del DCE.
Un'altra distinzione va fatta in base al tipo di connettore che puo' essere del tipo DB9 9 pin o DB25 25 pin , vediamo le piedinature di entrambi:. In base alle distinzioni sopracitate vediamo quindi come effettuare i collegamenti. If no flow control is implemented in the null modem connection, communication is only possible at speeds at which it is sure the receiving side can handle the amount information even under worst case conditions.
When we look at the connector pinout of the RS port, we see two pins which are certainly used for flow control. A simple but useful mechanism allowing flow control in one direction.
In the past, this was true. Modem speeds of baud were common and baud was seen as a high speed connection. For further control of the information flow, both devices have the ability to signal their status to the other side. These signals are in one direction only. It is not used directly for flow control, but mainly an indication of the ability of the modem device to communicate with its counter part. This signal indicates the existence of a communication link between two modem devices.
How to use the handshaking lines in a null modem configuration? In that situation, only the data lines and signal ground are cross connected in the null modem communication cable. All other pins have no connection. An example of such a null modem cable without handshaking can be seen in the figure below. If you read about null modems, this three wire null modem cable is often talked about. Yes, it is simple but can we use it in all circumstances?
These signals normally define the ability of the other side to communicate. As they are not connected, their signal level will never go high. This might cause a problem.
If the software on both sides is well structured, the RTS output is set high and then a waiting cycle is started until a ready signal is received on the CTS line. This causes the software to hang because no physical connection is present to either CTS line to make this possible. This does however not mean, that this null modem cable is useless.
Communication links like present in the Norton Commander program can use this null modem cable. This null modem cable can also be used when communicating with devices which do not have modem control signals like electronic measuring equipment etc. As you can imagine, with this simple null modem cable no hardware flow control can be implemented.
The simple null modem cable without handshaking shows incompatibilities with common software. The main problem with this cable is that there is a possibility for the software to hang if it checks the modem signal lines in a proper way. To overcome this problem and still be able to use a cheap null modem communication cable with only three lines in it, a fake null modem cable layout has been defined.
The null modem cable with loop back handshaking resulted from this. The main purpose of this null modem cable is to let well defined software think there is handshaking available, with a null modem cable which has no provisions for it. Consider first the DSR signal pin 6. This input indicates that the other side is ready to start communicating.
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