PC Hardware in a Nutshell, 2nd Edition

23.4 Parallel Connectors and Cables

IEEE 1284-1994 defines both the electrical and physical interface for cables and connectors. Cable quality is critical for IEEE 1284, because various IEEE 1284 modes support much higher transmission speeds than SPP.

23.4.1 Parallel Connectors

Traditional parallel cables use a DB25M connector for the PC end and a male, 36-pin, 0.085" centerline Champ connector with bale locks (commonly called a Centronics C36M) for the printer. The IEEE 1284-1994 specification allows these two traditional connectors to be used as before. It designates the DB25M the IEEE 1284-1994 Type A Connector and the C36M the IEEE 1284-1994 Type B Connector. IEEE 1284 also defines a new type of parallel connector, called the 1284-1994 Type C Connector, which uses a 36-pin, 0.050" centerline mini-connector with clip latches, and is usually called a mini-Centronics connector. Printer cables are now available that use these connectors in many combinations.

23.4.2 PC-to-Peripheral Parallel Cables

It used to be that a printer cable was a printer cable. Not anymore. Printer cables now come in a variety of types, which use different connectors and pinouts. The good news is that you can still use any printer cable to connect a PC to a printer—as long as the connectors physically fit—and that connection will work in some fashion. The bad news is that using an old printer cable may cripple the performance and functionality of the link.

When you buy a new parallel cable—which you should if you are now using an older cable to connect a recent port to a recent peripheral—make sure it's labeled "IEEE 1284-1994 Compliant." Table 23-1 through Table 23-4 show the pin connections for the standard IEEE 1284 cables you are likely to need. To ensure optimum parallel performance, use an IEEE 1284 cable with connectors appropriate for your PC parallel port and the peripheral to be connected.

Table 23-1 shows the pinouts for a standard SPP 25-wire Centronics C36M-to- DB25M parallel printer cable, including signal polarities and directions. The missing C36M pins are not connected. The original IBM Parallel Cable and some inexpensive currently-available cables use only 18 wires, using a single wire to tie DB25M pins 18 through 25 to C36M pins 19 through 30 and 33. These 18-wire cables may not work in all applications, notably with OS/2.

Table 23-1. The pinouts for a standard SPP 25-wire Centronics C36M-to-DB25M parallel printer cable

C36M	DB25M	Description		C36M	DB25M	Description
1	1	-nStrobe (out)		14	14	-nAutoFd (out)
2	2	+Data Bit 0 (out)		19	19	-Data Bit 1 Return (GND) (in)
3	3	+Data Bit 1 (out)		21	20	-Data Bit 2 Return (GND) (in)
4	4	+Data Bit 2 (out)		23	21	-Data Bit 3 Return (GND) (in)
5	5	+Data Bit 3 (out)		25	22	-Data Bit 4 Return (GND) (in)
6	6	+Data Bit 4 (out)		27	23	-Data Bit 5 Return (GND) (in)
7	7	+Data Bit 5 (out)		29	24	-Data Bit 6 Return (GND) (in)
8	8	+Data Bit 6 (out)		30	25	-Data Bit 7 Return (GND) (in)
9	9	+Data Bit 7 (out)		31	16	-nInit (out)
10	10	-nAck (in)		32	15	-nFault (in)
11	11	+Busy (in)		33	18	-Data Bit 0 Return (GND) (in)
12	12	+PE (in)		36	17	-nSelectIn (out)
13	13	+Select (in)

Table 23-2 shows the pinouts for an IEEE 1284 A-to-B adapter cable, used to connect a DB25M, Type A EPP, ECP, or IEEE 1284-compliant PC parallel port to a peripheral with a Centronics, C36M Type B connector. Note that DB25M pins 1 through 17 carry the same signals as the preceding cable, and that DB25M pins 18 through 25 are similarly used for ground returns, although with slightly different definitions. Because it uses the same connectors as the SPP parallel cable described in the preceding table, the only way to differentiate this cable visually is to look for the "IEEE 1284-1994 Compliant" label.

Table 23-2. The pinouts for an IEEE 1284 A-to-B adapter cable

C36M	DB25M	Description		C36M	DB25M	Description
1	1	NStrobe		14	14	nAutoFd
2	2	Data Bit 0		19	18	nStrobe ground return
3	3	Data Bit 1		20, 21	19	Data Bits 0 & 1 ground return
4	4	Data Bit 2		22, 23	20	Data Bits 2 & 3 ground return
5	5	Data Bit 3		24, 25	21	Data Bits 4 & 5 ground return
6	6	Data Bit 4		26, 27	22	Data Bits 6 & 7 ground return
7	7	Data Bit 5		28	24	nAck, PE & Select ground return
8	8	Data Bit 6		29	23	Busy & nFault ground return
9	9	Data Bit 7		30	25	nAutoFd, nInit & nSelectIn ground return
10	10	NAck		31	16	nInit
11	11	Busy		32	15	nFault
12	12	PE		36	17	nSelectIn
13	13	Select

Table 23-3 shows the pinouts for an IEEE 1284 A-to-C adapter cable, used to connect a DB25M, Type A EPP, ECP, or IEEE 1284-compliant PC parallel port to a peripheral with a mini-Centronics, Type C connector.

Table 23-3. The pinouts for an IEEE 1284 A-to-C adapter cable

Type C	Type A	Description		Type C	Type A	Description
1	11	Busy		14	16	nInit
2	13	Select		15	1	nStrobe
3	10	NAck		16	17	nSelectIn
4	15	NFault		17	14	nAutoFd
5	12	PE		19, 22	23	Busy & nFault ground return
6	2	Data Bit 0		20, 21 & 23	24	nAck, PE & Select ground return
7	3	Data Bit 1		24 & 25	19	Data Bits 0 & 1 ground return
8	4	Data Bit 2		26 & 27	20	Data Bits 2 & 3 ground return
9	5	Data Bit 3		28 & 29	21	Data Bits 4 & 5 ground return
10	6	Data Bit 4		30 & 31	22	Data Bits 6 & 7 ground return
11	7	Data Bit 5		32, 34 & 35	25	nAutoFd, nInit & nSelectIn ground return
12	8	Data Bit 6		33	18	nStrobe ground return
13	9	Data Bit 7

Table 23-4 shows the pinouts for an IEEE 1284 C-to-B adapter cable, used to connect a mini-Centronics, Type C PC parallel port to a peripheral with a Centronics, Type B connector. This is an unusual cable for now, but will become more common if and when PC parallel ports with IEEE 1284 Type C connectors become more common. Because parallel ports are being de-emphasized in new motherboards and PCs, that day may well never arrive.

Table 23-4. The pinouts for an IEEE 1284 C-to-B adapter cable

Type C	Type B	Description		Type C	Type B	Description
1	11	Busy		19	29	Busy ground return
2	13	Select		20	28	Select ground return
3	10	nAck		21	28	nAck ground return
4	32	nFault		22	29	nFault ground return
5	12	PE		23	28	PE ground return
6	2	Data Bit 0		24	20	Data Bit 0 ground return
7	3	Data Bit 1		25	21	Data Bit 1 ground return
8	4	Data Bit 2		26	22	Data Bit 2 ground return
9	5	Data Bit 3		27	23	Data Bit 3 ground return
10	6	Data Bit 4		28	24	Data Bit 4 ground return
11	7	Data Bit 5		29	25	Data Bit 5 ground return
12	8	Data Bit 6		30	26	Data Bit 6 ground return
13	9	Data Bit 7		31	27	Data Bit 7 ground return
14	31	nInit		32	30	nInit ground return
15	1	nStrobe		33	19	nStrobe ground return
16	36	nSelectIn		34	30	nSelectIn ground return
17	14	nAutoFd		35	30	nAutoFd ground return
18	-	Host Logic High		36	18	Peripheral Logic High

23.4.3 PC-to-PC Parallel Cables

NT does not support direct parallel connections, but Windows 9X Direct Cable Connection can be used to establish a parallel-to-parallel link between two PCs. You can use three types of DB25M-to-DB25M cables for a DCC parallel connection, designated by Microsoft as follows:

Standard cable

The Standard cable, shown in Table 23-5, is also called a Basic 4-bit cable, LapLink cable, or InterLink cable. This is the slowest parallel DCC cable, but can be used to link computers with any types of parallel port, including dissimilar ports on the two computers. Expect throughput of 40 to 70 KB/s when using one of these cables—painfully slow, but still about ten times the speed of DCC over a serial connection.

Table 23-5. Standard cable for use with InterLink or Direct Cable Connection

DB25M	DB25M	Connection description
2	15	Data bit 0 (Active when high)
3	13	Data bit 1 (Active when high)
4	12	Data bit 2 (Active when high)
5	10	Data bit 3 (Active when high)
6	11	Data bit 4 (Active when high)
10	5	Acknowledge (Active when low)
11	6	Busy (Active when high)
12	4	Out of Paper (Active when high)
13	3	Select (Active when high)
15	2	Error (Active when low)
25	25	Ground to Ground

Extended Capabilities Port cable

The Extended Capabilities Port cable, shown in Table 23-6, is also called an ECP cable. This cable can be used to link computers that both have ECP parallel ports (including IEEE 1284 ports in ECP Mode) installed and enabled. It provides much faster throughput than the standard cable—500 KB/s or more, depending on the ports.

Table 23-6. ECP cable for use with InterLink or Direct Cable Connection

DB25F	DB25F	Connection description
1	10	nStrobe to nAck
2 - 9	2 - 9	Data to Data (straight through)
10	1	nAck to nStrobe
11	14	Busy to nAutoFwd
12	16	pError to nInit
13	13, 17	Select to Select and nSelect
14	11	nAutoFwd to Busy
15	17	nFault to nSelectIn
16	12	nInit to pError
17	15	nSelectIn to nFault
18 - 25	18 - 25	Ground to Ground (straight through)

Universal Cable Module cable

The Universal Cable Module cable, also called a UCM cable, can be used to link two computers that have different types of parallel ports. It's not really just a cable, because it includes active electronic components that automatically optimize throughput between PCs with differing port types. This cable can be very useful when both PCs do not have ECP-capable parallel ports and you want to get the highest performance available for the combination of hardware being used—for example, duplicating a standard PC configuration to multiple PCs when those PCs do not have network cards, or backing up a notebook computer to a desktop system.

The only source we've found for this cable is Parallel Technologies (http://www.lpt.com). Their Universal Fast Cable costs $70, and includes monitoring software. When used to connect two ECP or two EPP ports, this cable can provide throughput of about 500 KB/s, within striking range of a 10 Mb/s Ethernet link. Note, however, that there is no real reason to buy this cable if all your parallel ports are ECP-capable—you can simply use the ECP cable described above. The benefit of this cable is that it automatically detects the port types in use and optimizes throughput for them.