RS-232 Null modem cable

What is a null modem cable?

The difference between a modem cable and a null modem cable is easy to understand: A modem cable usually connects a computer to a modem. A null modem cable, on the other hand, connects two computers to each other, or a computer to a printer, point-of-sale system, etc. Generally speaking, a null modem cable connects (RS-232) machine to (RS-232) machine; there is no connection to a modem.

To meet the different requirements, there are different variants of null modem cables. In order the customer can make a fitting null modem connection with the RS-232 devices available in each case, Delock offers three different solutions. In the following, read more about null modem (cable) features in general, as well as the three null modem cable solutions by Delock.

Content

Figure 1: Wiring of a RS-232 interface on a Sub-D9 connection

Features null modem

1. Common to all null modem cables is the presence of the crossed RXD and TXD data lines in order to be able to send data back and forth between the RS-232 devices (see figure 2).
2. An essential difference between the different null modem cable variants is the control of the data transmission. On the one hand, control can be achieved by different pin assignments (RTS / CTS and DTS / DTR) or on the software side in the data flow control settings (No control, hardware, Xon / Xoff *). This is also referred to as "handshaking".
3. The two inputs RI and DCD, actually a relic from the modem era, have new tasks. Nowadays, for example, these are used to provide additional 5V/12V power supplies for RS-232 devices. All Delock null modem cables offer this possibility.
4. Shielding and ground are available at all Delock RS-232 cables.

Figure 2: GND (Ground), the simplest variant of PINOUT: 2 - 3 , 3 - 2 , 5 - 5

Three Delock null modem cables

Null modem cable variant 1: The classic

This null modem cable is very widely used and is suitable for most applications. The RS-232 control lines at this cable are largely wired, so that both modern and older RS-232 devices can be connected. CTS / RTS are available for data flow control. This can also be done with DTR / DTS, in which case the input signal DCD can also be used optionally.

Figure 3: PINOUT: 1 + 6 - 4 , 2 - 3 , 3 - 2 , 4 - 1 + 6 , 5 - 5 , 7 - 8 , 8 - 7

Overview of Delock products with null modem variant "The Classic“

Null modem cable variant 2: Full Handshaking

The Full Handshaking null modem cable is, on the one hand, suitable to connect computers with diverse RS-232 devices, e.g. IOT machines and, on the other hand, to program setup boxes or satellite receivers and so on. The cable allows the control of data transmission with RTS / CRS and with DTR / DTS. Furthermore, for example POS systems can additionally be supported with power via RI on pin 9. So this null modem cable provide further application possibilities.

Figure 4: PINOUT: 2 - 3 , 3 - 2 , 4 - 6 , 5 - 5 , 6 - 4 , 7 - 8 , 8 - 7 , 9 - 9

Overview of Delock products with null modem variant "Full Handshaking"

Null modem cable variant 3: RTS / CTS Auto Control

With this null modem cable, special requirements on sequence control can be meet. Some RS-232 programs expect a certain sequence of operations to be followed between RS-232 devices. For it, this cable automatically activates the transmit and receive readiness at the control lines RTS / CTS and DCD of both connected RS-232 devices. In the further process, if required, DTR / DTS can also be used for control.

Figure 5: PINOUT: 7 + 8 - 1 , 2 - 3 , 3 - 2 , 4 - 6 , 6 - 4 , 5 - 5 , 1 - 7 + 8

Overview of Delock products with null modem variant "RTS / CTS Auto Control"