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Null-Modem Serial Cables



Q: What cable do I need to connect Amiga Explorer over serial ports?



A: A standard "null-modem serial cable" (optionally attached to the PC via a USB serial interface), or a Bluetooth serial adapter. The null-modem cable needs to be wired for "full handshaking" (not "loopback handshaking" or "partial handshaking"). These cables are architecture-neutral, i.e. they are the same for PC, Amiga and other systems.

The Amiga generally has a 25-pin serial port connector (on the back), whereas PCs use both 9-pin and 25-pin connectors. If the PC does not have a serial port, you can use an inexpensive USB serial adapter.

On both Amiga and PC systems, be careful not to confuse the serial port (male connector) with the parallel port (female connector). Also, the Amiga has two 9-pin mouse/joystick connectors on the side, which are not serial ports.

Belkin, for example, lists a high-quality, universal (9-pin and 25-pin connectors on both sides, i.e. a total of four connectors) null-modem cable as model number F3X171-10 ("Pro Series File Transfer - PC to PC Cable - Serial"). QVS offers a similar "Universal Serial File Transfer Cable" as part number CC327-10, which we also tested successfully. Larger computer stores often carry at least one of these two brands. In our experience prices for such "universal" null-modem cables (i.e. those with four connectors) vary between about $10 and $30, even for the same product sold at different locations. Regular null-modem cables tend to sell in the $5-$7 range.

We did receive a few reports about commercial cables that were not wired properly. Please refer to the section on null-modem cables in the Amiga Explorer documentation for detailed and complete information on this topic, including non-standard Amiga serial ports. The following diagrams show simple schematics for three common null-modem wiring configurations (choose the one that applies based on your combination of PC/Amiga connectors).


Null-modem wiring (DB-25 to DB-25)

DB-25 connectors on both ends


Null-modem wiring (DB-9 to DB-25)

DB-9 connector on one end, DB-25 on the other end


Null-modem wiring (DB-9 to DB-9)

DB-9 connectors on both ends


If you purchased a null-modem cable, and it does not work, please compare it with the schematics in the Amiga Explorer documentation. No parts in this wiring are "redundant".

If you need to use a "gender changer" to attach a null-modem serial cable to the serial port on the back of your computer, it probably means that the cable is not a null-modem serial cable. Null-modem cables should always have female connectors on both sides, and all computers should have male connectors for their serial ports. (A known exception to this are some early Amiga 1000 models. A small number of these had female serial port connectors. If you are in doubt check the indications appearing near the connectors.)

The serial port is normally used for modems, and may have a telephone symbol next to it. The parallel port (which is not used with Amiga Explorer) is normally used for printers, and may have a printer symbol next to it.

Warning: the 25-pin female connector on the back of many computers is a parallel port (not a serial port). While a null-modem serial cable will not attach to it (as it also uses female connectors), certain other combinations of cables and gender changers can in theory be used to try to connect a parallel port on one computer with the serial port of another computer. This should never be attempted, as it may damage one or both computers.

For some applications, including terminal software when no hardware handshaking is used, very simple null-modem cables with three wires are sufficient to exchange data (although not in the most reliable and efficient way). This is accomplished by connecting GND (system ground) on both sides, and cross-connecting TxD (transmit data) with RxD (receive data). Amiga Explorer, like many other programs, additionally uses RTS and CTS for hardware handshaking, and DTR and DSR for each side to inform the other side whether it is ready to process data or not. This ensures, for example, that if there is an overflow in the serial port's input data buffer, the other side stops transmitting, and waits until the buffer is ready to receive more data.

Amiga Explorer requires a complete null-modem cable, with 7 wires. High quality commercial cables follow this specification, but lower quality and "do-it-yourself" cables sometimes differ. If the wires are not connected properly, Amiga Explorer will not work. In some cases, including proper connections but poor quality or too long cables, Amiga Explorer may transfer short files, but it will fail with longer files (in one or both directions). In other cases, the software will not work at all. The Amiga serial port also uses non-standard signals (see the Amiga Forever documentation), which may damage the equipment on the other side. This cannot happen with a properly wired null-modem cable.

The pins for all DB-9/25 connectors are numbered in such a way that each pin number always corresponds to the same pin number on the other side. For example, pin 1 always connects to pin 1, and is always numbered "1" in all of the above illustrations and tables, regardless of whether it is "male" or "female", or viewed from "inside" or "outside". All pins along this electrical path are numbered "1".

It is very important that the pins which on the different Amiga models supply power (9, 10, 14, 21, 23) be not connected to any active pins on the PC. Also, the two different grounds (shield ground and system ground) should not be connected with each other.

Cables longer than 20 meters (65 feet) should be used with additional care. Special low capacitance cables are better suited for longer distances. Shielded cables and connectors should be used if the cables are close to electrical equipment, which could add noise to the signals. The cable shielding should be connected to the shielding of one of the two sockets. Lower baud rates may help reduce transmission problems over long and/or disturbed lines. The use of additional connectors, adapters and extension wires should be kept to a minimum, as each additional component tends to reduce the quality of the connection.

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Article Information
Article ID: 13-105
Platform: Windows
Products: Amiga Explorer, Amiga Forever
Additional Keywords: TIA 232, EIA 232, RS-232
Last Update: 2019-12-15
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