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.
* Xon / Xoff data flow control is provided for RXD / TXD.
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“

Item no. Cable length Name Connectors
86614 0.5 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
86608 1 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
86605 2 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
86606 3 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
86620 4 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
86607 5 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
86604 10 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
86615 1 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Male to female
86616 2 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Male to female
86617 3 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Male to female
84077 1.8 m Serial cable RS-232 D-Sub9 • null modem Female to female
84169 3 m Serial cable RS-232 D-Sub9 • null modem Female to female
84250 5 m Serial cable RS-232 D-Sub9 • null modem Female to female

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"

Item no. Cable length Name Connectors
87308 0.5 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
87278 1 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
87472 2 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
87516 5 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
87525 10 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female

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"

Item no. Cable length Name Connectors
87788 0.5 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
87786 1 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
87785 2 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
87784 5 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
87783 10 m Serial cable RS-232 D-Sub9 • null modem • narrow plug housing Female to female
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