Null Modem Serial Cables
The Amiga Explorer software is designed to share data between an Amiga and one or more PCs using either TCP/IP (over Ethernet or another interface), or directly over a null-modem cable connection. The null-modem connection is the simplest and least expensive to set up, if no other connection is already in place (e.g. Ethernet with TCP/IP). A standard null-modem serial cable can be purchased wherever computers and accessories are sold, and shouldn't cost more than any other serial cable.
Normally, a serial cable connects a computer ("DTE", from Data Terminal Equipment) with a modem ("DCE", from Data Communications Equipment). A "null modem" cable has certain wires crossed in such a way that a computer can communicate with another computer, with each system appearing to the other as if it were a modem (when instead there is no modem between them, hence the name "null modem").
Serial port connectors exist with either 25 or 9 pins, as shown in the following tables.
| PC/Amiga 25 Pin Connector | ||||
|---|---|---|---|---|
| Pin | Name | Description | ||
 DB-25 plug ("male"), standard on the back of all Amiga and many PC systems (DTE)
DB-25 socket ("female"), used in null modem cables, and in modems (DCE) |
1 | SHIELD | Shield Ground | |
| 2 | TxD | Transmit Data | ||
| 3 | RxD | Receive Data | ||
| 4 | RTS | Request to Send | ||
| 5 | CTS | Clear to Send | ||
| 6 | DSR | Data Set Ready | ||
| 7 | GND | System Ground | ||
| 8 | DCD | Data Carrier Detect | ||
| 9 | +12V | +12 Volt | ||
| 10 | -12V | -12 Volt | ||
| 11 | AUDO | Left Audio Out | ||
| 12 | SPEED | Speed Indicate | ||
| 13 | - | Unused | ||
| 14 | -5V | -5 Volt | ||
| 15 | AUDO | Left Audio Out | ||
| 16 | AUDI | Right Audio In | ||
| 17 | EB | Buffered Port Clock 716 kHz | ||
| 18 | AUDI/INT2 | Right Audio In / Interrupt 2 | ||
| 19 | - | Unused | ||
| 20 | DTR | Data Terminal Ready | ||
| 21 | +5V | +5 Volt | ||
| 22 | RI | Ring Indicator | ||
| 23 | +12V | +12 Volt | ||
| 24 | C2 | Clock 3.58 MHz | ||
| 25 | RESB | Buffered System Reset | ||
| Black = Standard, Green = Amiga 1000, Red = Other Amiga systems | ||||
| PC 9 Pin Connector | ||||
|---|---|---|---|---|
| Pin | Name | Description | ||
DB-9 plug ("male"), used on Amiga and PC multiserial boards, and standard on many PCs (DTE)
DB-9 socket ("female") , used in null modem cables, and in modems (DCE) |
1 | DCD | Data Carrier Detect | |
| 2 | RxD | Receive Data | ||
| 3 | TxD | Transmit Data | ||
| 4 | DTR | Data Terminal Ready | ||
| 5 | GND | System Ground | ||
| 6 | DSR | Data Set Ready | ||
| 7 | RTS | Request to Send | ||
| 8 | CTS | Clear to Send | ||
| 9 | RI | Ring Indicator | ||
All Amiga systems by Commodore and by Amiga Technologies were manufactured using DB-25 serial port connectors. Third-party Amiga multiserial boards, which add one or more serial ports to the Amiga's single port, use either DB-25 or DB-9 connectors (as well as other, non-standard connectors, which are also described here). PCs usually have two serial ports (referred to as "COM1" and "COM2"), which may use either DB-9 or DB-25 connectors, or one of each type (e.g. DB-9 for the mouse, and DB-25 for the modem). 9 pins/wires are sufficient for most of today's PC applications of serial ports (to connect modems, computers, mice, printers, terminals, etc.), so it does not matter whether a DB-9 or DB-25 connector is used. Different types of adapters are available to convert from one format to the other.
Depending on the type of connector used on each side, the null modem cable can have DB-25 connectors on both ends, or DB-9 connectors on both ends, or DB-9 on one end and DB-25 on the other. In either case, both ends of the cable are always "female".
The following tables describe some other types of Amiga connectors. If necessary, null modem cables can be built for these connectors by wiring the pins corresponding to the standard PC/Amiga connector pins.
| GVP I/O Extender Connector | ||||
|---|---|---|---|---|
 |
Pin | Name | Description | |
| 1 | DCD | Data Carrier Detect | ||
| 2 | RxD | Receive Data | ||
| 3 | TxD | Transmit Data | ||
| 4 | DTR | Data Terminal Ready | ||
| 5 | GND | System Ground | ||
| 6 | DSR | Data Set Ready | ||
| 7 | RTS | Request to Send | ||
| 8 | CTS | Clear to Send | ||
| 9 | RI | Ring Indicator | ||
| Commodore A2232 Multiport Serial Card Connector | ||||
|---|---|---|---|---|
 |
Pin | Name | Description | |
| 1 | TxD | Transmit Data | ||
| 2 | RxD | Receive Data | ||
| 3 | RTS | Request to Send | ||
| 4 | CTS | Clear to Send | ||
| 5 | DSR | Data Set Ready | ||
| 6 | GND | System Ground | ||
| 7 | DCD | Data Carrier Detect | ||
| 8 | DTR | Data Terminal Ready | ||
For some applications, 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, 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 below), which may damage the equipment on the other side. This cannot happen with a properly-wired null modem cable.
The following drawings explain in detail the different types of wiring of the various null modem cables.
DB-25 connectors on both ends
DB-9 connector on one end, DB-25 on the other end
DB-9 connectors on both ends
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.