AMX TXC16 Plus User Manual download pdf (Page 14)

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? There is a problem, if either of the two devices checks the DSR

or CD inputs. These signals normaly 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.

The same holds for the RTS/CTS handshaking sequence. 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. The only type of communication which is allowed on such a null modem

line is data-only traffic on the cross connected RX/TX lines.

This does however not mean, that this cable is useless. Communication links like present in the Norton

Commander program can use this cable. This 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 cable no hardware flow control can be implemented. The only way

to perform flow control is with software flow control using the XOFF and XON characters.

Null modem with loop back handshaking

Loop back handshaking

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. I.e. with this cable, good written programs will perform worse than badly

written programs.

To overcome this problem and still be able to use a cheap communication cable with only three lines in

it, a fake cable layout has been defined. The null modem cable with loop back handshaking resulted

from this.

The main purpose of this cable is to let well defined software think there is handshaking available, with

a cable which has no provisions for it.

Compatibility issues

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). This means, that the

software doesn't see the ready signal of the other device, but its own. 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. This is true for at least 99%

of all serial communication software. This implies that at least 99% of all serial communication software

is capable of faking its own DSR check with this cable.

The same trick is used with the CTS input. In the original use, RTS is set, and then CTS is checked

before starting the communication. By setting the RTS output (pin 7) the CTS input on the same

connector (pin 8) is receiving clearance immediately. There is no possibility of a software hangup

because of dangling RTS requests.

Other issues to consider

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AMX TXC16 Plus User Manual Page 14