Null modem circuits diagrams

To build this cable, you'll need a shielded cable with at least 15 conductors, and two DB male connectors. It is suggested that you attach the drain wire of the shield of your cable to the shield of the DB connector. Note - There are a number of cable diagrams for LapLink parallel cables on the web, many of which are contradictory. The above chart matches the pinout of a LapLink parallel cable measured by me. Null modem is a communication method to directly connect two DTEs computer, terminal, Several wiring layouts are in use because the null modem connection is not covered by the RS standard.

A null modem cable is a RS serial cable where the transmit and receive lines are crosslinked. In some cables there. Null modem cable DB9-DB9. Pinout and signals for building a serial RS nullmodem cable. 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. In the layout, the line is linked back to the DTR output pin 4. The same holds for the CD input pin 1. The assumption is, that if software has been written to check the DSR line to test communication availability, it will probably also set the DTR output to indicate its own state. The same trick is used with the CTS input.

There is no possibility of a software hangup because of dangling RTS requests. The null modem cable with loop back handshaking is often advised as the best low cost available null modem cable. But, is it really so good? The simple null modem cable without handshaking has the disadvantage that it does not permit proper written software to communicate with it.

Software which is aware of the lack of handshaking signals can however use it without problems. The null modem cable with loop back handshaking can be used with more software, but it has no functional enhancements over the simple cable!

If the software is designed for using hardware flow control it seems to work with this null modem cable, but on unpredictable moments, data loss may occur. This means that the null modem cable allows communication as long as no flow control is needed, but when data speeds reach the limit the receivers can handle, communication may stop immediately without an assignable reason. Therefore, although this null modem cable is cheap and easy to make, use it with care! The simple null modem cable and the null modem cable with loop back handshaking are useful, but have no provisions for hardware flow control.