AMX TXC16 Plus User Manual download pdf (Page 15)

The null modem cable with loop back handshaking is often advised as the best low cost available cable.

But, is it really so good? The simple 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 cable with loop back handshaking can be used with more software, but it has no functional

enhancements over the simple cable! There is no way both devices can control data flow, other than by

using XON/XOFF handshaking. If the software is designed for using hardware flow control it seems to

work with this cable, but on unpredictable moments, data loss may occur. This means that the 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 cable is cheap and easy to make, use it with care! Despite these warnings, this type of

cable has been successfully used between Windows 95/98 computers with a Direct Cable Connection.

Null modem with partial handshaking

Partial handshaking

The simple null modem cable and the cable with loop back handshaking are useful, but have no

provisions for hardware flow control. If it is absolutely necessary that hardware flow control is used, the

null modem with partial handshaking can be an alternative.

Compatibility issues

This cable is the best of two worlds. There is the possibility of hardware flow control without being

incompatible with the original way flow control was used with DTE/DCE communication. Let us first

consider the RTS/CTS flow control lines present on pins 7 and 8. As with the loop back cable, these

signals are not connected to the other device, but directly looped back on the same connector. This

means, that RTS/CTS flow control is allowed to be used in the software, but it has no functional

meaning. Only when the software at the other side checks the CD signal at pin 1, the RTS information

will reach the other device. This would however be only the case in specifically developed software

which uses the CD input for this purpose.

More important however is the cross connection of the DSR (pin 6) and DTR (pin 4) lines. By cross

connecting these lines, their original function is simulated pretty well. The DTR output is used to signal

the other device that communication is possible. This information is read on the DSR input, the same

input used for this purpose with modem communication. Because of this cross connection, the DTR

output line can be used for simple flow control. Incomming data is allowed when the output is set, and

blocked if the output is not set.

Software using only the RTS/CTS protocol for flow control cannot take advantage of the partial

handshaking cable. Most software however will also check the DSR line and in that case, using the null

modem cable with partial handshaking, the best possible hardware flow control can be achieved which

is still compatible with the original use with modems.

Null modem with full handshaking (explanation)

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AMX TXC16 Plus User Manual Page 15