One of the most common problems encountered in premises data communications concerns the interfaces of the data equipments being connected. Of course, you may be totally oblivious even to the existence of interfaces. Many data communications users never think about them. Letís get this out of the way right at the start and answer the simple query; What exactly is an interface?

Simpler yet is the response. An interface is just the point of connection on a data equipment unit. It is the location where data comes into and out of the equipment. Connecting two (2) different data equipment units together by their interfaces allows data and control information to be interchanged by them.

For such a simple definition interfaces provide considerable "meat" for discussion. Yet, this chapter should cover everything you need to know about them. This includes mechanical connectors, electrical signals, common problems and the area of interface conversion.


It will be worthwhile describing what interfaces look like from the physical-mechanical view. Take a look at the back of some data equipment unit, a terminal, a printer or a computer. You will see an interface. Unfortunately, we cannot say that this is a typical or a standard interface because, in fact, there is no such thing as a typical or a standard interface. However, in order to move our discussion along we can talk about the most common interfaces encountered.

The most ubiquitous physical interface is the DB25 Connector. This is a 25 pin, female connector which allows data and control signals to enter or exit a data equipment unit. Most other interfaces are also female. You will also find DB37, DB15, DB9, even larger rectangular 34 pin, female connectors. There are exceptions to this rule of generally having female connectors. First, there is the DEC world where you will find male connectors. Secondly, there is the world of the Personal Computer where the IBM PC AT and PC 386 both have 9 pin male connectors for the serial data communications port. Finally, certain equipment designers use modular phone connectors where there are three sizes; 4,6 or 8 pins.


Equally important is to understand the electrical function of an interface. The voltages on the pins of an interface are used to represent both the data and control signals going into and out of the data equipment.

These signal voltages may either be single ended or differential. When single ended, they are measured with respect to a common ground in the interface. When differential, the voltages are generally differences between voltages that appear on two signal pins.

Voltages may also either be unipolar or bipolar. When unipolar, the voltage may be either a positive or negative relative to the ground. When bipolar, pin voltages will be either positive or negative depending on data. The most common interface is called EIA-232C (formerly RS-232C) or the European counterpart V.24. Many people still refer to "RS-232C" today even though this standard has been accepted and is officially "EIA-232C." Even at Telebyte we use "RS" and "EIA" interchangeably. This is the price for waiting so long to approve a recommended standard. Recently, EIA-232C has been "replaced" with EIA-232D. The differences between them are minor. The maximum distance for EIA-232C is specified as "50 feet." EIA-232D is specified by having a maximum cable capacitance of 2500 pF. This can be converted to a maximum length by dividing 2500 by the capacitance per foot of the EIA-232D cable.

"EIA" stands for "Electronic Industries Association" the principal data communications industry standards organization within the United States. "RS" stands for "Recommended Standard" of the EIA before formal adoption. "V" stands for the "fifth"section of the C.C.I.T.T. specifications. C.C.I.T.T. is the principal datacom industry standards organization in Europe. It is the initials of a French name which can be translated as Consulting Committee for International Telephony and Telegraphy.

In Figure 3 we show the mechanical layout and the pin definitions for interfaces which you may typically encounter. These include V.35, EIA-232C, Centronics and RS-449.

Pin Signal Pin Signal
A Chassis Ground B Signal Ground
C Request to Send D Clear to Send
E Data Set Ready F Receive Line Signal Detect
H Data Terminal Ready J Ring Indicator
P Transmitted Data (Signal A) R Recieved Data (Signal A)
S Transmitted Data (Signal B) T Received Data (Signal B)
U Terminal Timing V Receive Timing A
W Terminal Timing X Receive Timing
Y Transmit Timing AA Transmit Timing

Figure 3a: V.35 Interface

Besides these interfaces you may come across others in dealing with your premises data communications problems. EIA-232 has an upper data rate limit of 19.2 KBaud. EIA-530 is a signal assignment which supports data rates above this but using the same mechanical specifications (connector). If you are dealing with data equipment in a military environment you may encounter "flavors" of the MIL-STD-188 interface (i.e. 188C, 188-114 Type 1,188-114 Type 2). These closely parallel EIA-232 but with a few deviations such as inverted logic for data signals and +/- 6 Volts for data control. Grocery stores with bar code readers at cash registers and building security systems with badge readers and intrusion detection devices may use polling systems employing the RS-485 interfaces.

If you would like to learn about the detail of these (and other) interface standards order a copy of Telebyteís "Data Communications Standards Library." Its a worthwhile reference and costs only $ 49.95.

Description From
1 AA Protective Ground

2 BA Transmitted Data
3 BB Received Data *D
4 CA Request to Send
5 CB Clear to Send *C
6 CC Data Set Ready *C
7 AB Signal Gnd/Common Return

8 CF Rcvd. Line Signal Detector *C
12 SCF Secondary Rcvd. Line Sig. Detector *C
13 SCB Secondary Clear to Send *C
14 SBA Secondary Transmitted Data
15 DB Transmitter Sig. Element Timing *T
16 SBB Secondary Received Data *D
17 DD Receiver Sig. Element Timing *T
19 SCA Secondary Request to Send
20 CD Data Terminal Ready
21 CG Sig. Quality Detector *C
22 CE Ring Indicator *C
23 CI Data Sig. Rate Selector (DCE) *C
23 DA Transmitter Sig. Element Timing
Signal Type: D=Data, C=Control, T=Timing
Note: On the DB25 connector that is commonly used for RS232:
      Pins 9 and 10 are reserved for Data Set Testing.
      Pins 11, 18 and 25 are undefined.
      Pin 23 may be defined as CI or CH.

Figure 3b: Interface Specification for EIA-232C

Pin Signal Pin Signal
1 Data Strobe 19 (R) Data Strobe
2 Data Bit 1 20 (R) Data Bit 1
3 Data Bit 2 21 (R) Data Bit 2
4 Data Bit 3 22 (R) Data Bit 3
5 Data Bit 4 23 (R) Data Bit 4
6 Data Bit 5 24 (R) Data Bit 5
7 Data Bit 6 25 (R) Data Bit 6
8 Data Bit 7 26 (R) Data Bit 7
9 Data Bit 8 27 (R) Data Bit 8
10 Acknowledge 28 (R) Acknowledge
11 Busy 29 (R) Busy
12 Paper End 30 (R) Paper End
13 Select 31 Input Prime
14 Supply Ground 32 Fault
15 OSCXT 33 Undefined
16 Logic Ground 34 Undefined
17 Chasis Ground 35 Undefined
18 +5V 36 Undefined

Figure 3c: Specification for Parallel Interface (Centronics)

Description From
Signaling Rate Indicator

Send Data
Send Timing


Receive Data
Request to Send

Receive Timing
Clear to Send



Local Loopback
Data Mode


Terminal Ready
Receiver Ready


Remote Loopback
Incoming Call



Signaling Rate Selector
Incoming Call
Test Mode
Signal Ground

Receive Common
Terminal in Service
Select Standby
Signal Quality


New Signal
Standby Indicator
Send Common


Signal Type: D = Data, C = Control, T = Timing
Note: On the DB37 connector that is commonly used for RS449; Pins 3 and 21 are undfined
B = Return

Figure 3d: Interface Specification for RS-449

Cables are used to connect the interface of one data equipment unit to another. But even the simple problem of cable connection presents situations of which you should be aware. For example, consider the need to connect a computer to a printer with the computer having a female interface and the printer having a female interface. In order to accomplish the connection you would need a cable with male connectors at both ends and consistent with both the computer and printer interfaces.


The typical premises data communication user is always encountering operational problems related to interfaces.

These problems are grouped as: 1) identity, 2) gender, 3) distance, 4) interface conversion and 5) media conversion.


To discuss the identity problem it will be worthwhile to introduce some nomenclature. Data processing equipment is usually referred to as data terminal equipment and abbreviated DTE. These equipments such as terminals, computers and printers are considered as a class separate from equipment used to transport or communicate data such as modems, line drivers and multiplexers. Such communication equipment is referred to as data communications equipment and abbreviated DCE. Whether or not an equipment unit is a DTE or a DCE has significance with regards to the interface. Interfaces are usually defined assuming that the equipment of which the interface is the debarkation point is in fact a DTE.

A typical misconception is that the pin of the interface labeled Transmit Data (TD) implicitly assumes that the equipment of which it is part is a DTE. Along with this the interface is defined assuming that the data coming out of the TD pin is supposed to go into a data communication equipment unit, a DCE. The interfaces of a DTE and DCE must mate consistent with this assumption. The TD pin of a DTE interface must mate with the TD pin of the DCE interface.

However, what happens if you want to connect not a DTE and a DCE but two DTEís? If you think this is a rare possibility then think again. Actually, it is quite a common situation. It occurs when you want to connect a computer and a printer. Now show me the facility where someone does not need a computer and a printer linked. The computer outputs data on the TD pin, Pin 2, of its interface. However, if a one-to-one cable is used a printerís receive data pin, RD, Pin3, can not be mated properly to capture this data. No need to worry. This problem can be solved quite easily. Just use a passive modem eliminator in order, effectively, to change the wiring between the pins of the interface so that the computerís TD pin mates with the printerís RD pin. The device is called a modem eliminator because, historically, people used a "Rube Goldberg" approach to deal with this problem. They inserted DCE types of devices between the two DTE devices needing to be connected in order to get the consistent mating of their plugs. That is, in the early days of premises data communications users set up a system where the connector went from the computer to a modem which was connected back-to-back to another modem and then to a printer. This approach is illustrated in Figure 4.

Figure 4: Early Solution to Unscramble Interfaces

This haphazard approach did accomplish the proper mating of the pins. Of course, with this you were using the modem in a very uneconomic, costwise ineffective, manner. It was being used just to solve an interface problem and not being used for what it was designed, for extending the distance - one of the other subjects that we are going to be discussing. Much of the internal electronics of the modem was just being wasted The modem eliminator shown in Figure 5 is a device which effects the pin conversion but without the complexity and cost of the modemís signaling capacity.

Figure 5: Present Modem Eliminator

2.3.2 GENDER

Gender mending sounds exotic but is really quite easy to describe. We have talked about the interfaces, possibly being female or male. Yet, what happens when one wants to connect equipment units directly together where both units only have male connectors?

To deal with this problem you can either use a connecting cable with a female interface connector at each end or you can use a device called a gender mender which is sometimes called a sex changer. This small device plugs into the interface on one of the equipment units and changes gender, either from male to female or female to male, to be consistent with the sex of the interface on the other equipment unit. You can use two connecting cables with the same gender connector on both ends. A low profile, gender changer is shown in Figure 6.

Figure 6: Low Profile Gender Changer


There is always concern with extending the distance between two data equipment units that you are trying to connect by joining their interfaces together with a cable. The various interface standards shown in Figure 3, as well as any other interface standard, are usually defined for signaling at a known rate at a specific distance between the two data equipment units to be connected. That is, the voltage levels on the various pins are guaranteed to interchange data and control signals reliably, provided that the cable interconnecting the two interfaces together is no more than a certain length. As mentioned previously, for the EIA-232C interface, this distance can be at most 50 feet at 9600 Baud. The distance specifications for common interfaces are provided in Table 1.

Distance and Data Rate Specifications for Typically Encountered Interfaces
EIA-232C 50 feet at 9600 Baud
RS-422 1,000 feet at 256 KBaud
6,000 feet at 19.2 KBaud
18,000 feet at 9.6 KBaud
All full duplex
RS-485 No actual distance/data rate specification
Usually quoted same as RS-422 but half duplex
V.35 Data rate in excess of 56 KBaud
Distance usually quoted same as RS-422
Table 1

Of course, users need to be concerned when the data equipment units being interconnected are separated by more than the maximum allowed distance.

In order to connect the units together and effect reliable interchange you must use either a special extender cable or resort to signal boosting devices such as line drivers (also called short haul modems or limited distance modems). More will be said about these devices in a later Chapter 4.


Here the concern with interfaces involves the interconnection of two equipment units that need to be connected but have two different interfaces. For example, one might want to connect a computer to a printer with the computer having an EIA-232C interface while the printer has a Current Loop interface. The problem of interconnecting the equipment where the interfaces are different is quite complex and presents a number of different solutions.

First, you must determine the minimum number of signals needed for the two devices to communicate. Obviously, TD and RD (transmit and receive data) must be interconnected through the interfaces but many of the control signals which come from the various pins of the interfaces may not be needed. You need to look at the details of the equipment documentation and determine the minimum number of control signals to be transferred. To bring this point into sharpest relief most data equipment users donít realize that, if they can use the X-ON/X-OFF protocol they only need pins 2, 3 and 7 of the EIA-232 connector, all other pins are extraneous.

Once the minimum number of signals is determined you must find an interface converter that accomplishes the necessary end conversion and verify that it has the capability to satisfy this transfer of the minimum number of control signals. Obviously there is a trade off in the cost of the interface converter between the number of signals it can transfer and its cost.

Always physically locate the converter so as to get the maximum benefit from the interface which has the ability to transmit signals over the greater distance. For example, in connecting an RS-232C based DTE to an RS-485 based DTE the conversion should be done as close as possible to the EIA-232C device so as to get the greater distance benefit of RS-485.

Interface Converter Selection Guide
Model Interface 1 Interface 2 No. of Signals Converted Physical Remarks
Type Conn. Type Conn.
62-1/2 RS-232 DB25 EIA-530 DB25 13 SA RS-422 Signals
63-2SA RS-232 DB25 RS-422 Term 2 SA TD & RD LED's
63-3/4 RS-232 DB25 RS-422 DB25 8 SA Very Popular
65AX RS-232 DB25 CLOOP* TERM 2 SA Active or Passive
121 RS-232 CARD RS-422 TERM 2 RM Dual Converters
122 RS-232 CARD 422/485 TERM 4 RM Dual Converters
234 RS-232 DB25 RS-232 DB25F 9 SA Modem Eliminator
235 V.35 34 Pin F V.35 34 Pin F 7 SA Modem Eliminator
236 G.703 RJ-45 V.35/RS-232 DB25 4 SA 64 KBPS G.703
242 RS-232 DB25 X.21 DB15 5 SA Power Stealing
243 RS-232 DB25 X.21 DB15 5 SA DTE/DCE & LCD
245 RS-232 DB25 422/485 DB25 2 SA Opto-Isolated
248 V.35 DB25 V.35/232 DB25 10 SA Opto-Isolated
253 RS-232 DB9 RS-422 TERM 2 SA Power Stealing
256 RS-232 DB25 RS-422 RJ11/TERM 2 SA TD Powered/LCD
260 RS-232 DB25 RS-422 TERM 2 SA Power Stealing
261 RS-232 DB25 RS-422 DB25 2 SA DG Compatible
263 RS-232 DB25 RS-422 TERM 2 SA Power Stealing
265 RS-232 DB25 RS-422 TERM 2 SA TD Powered
267-12 RS-232 DB25 EIA530 DB25** 13 RM 10CATI,3 CAT II
268 RS-232 DB25 RS-232 DB25 4 SA Opto-Isolated
272 RS-422 DB25 Fiber ST 2 SA Full Duplex
276 RS-485 DB25 Fiber ST 2 SA Half Duplex
279 Fiber Multimode Fiber Singlemode 2 SA DC-2.5 MBPS
281 RS-422 DB25 RS-422 DB25 4 SA Opto-Isolated
282 RS-232 DB25 RS-232 DB25 8 SA Opto-Isolated
284 PC BUS CARD RS-422 DB25 10 CARD Supports IBM PC
285 RS-232 DB25 RS-422/485 TERM 2 SA Select 422 or 485
287 RS-232 DB9 RS-422 RJ-12 2 SA 4 KV Isolation
290 RS-232 DB25 422/485 RJ-11 2 SA 16 422/485
365 RS-232 DB25 RS-422/485 --- 2 SA AC Powered
366 RS-232 DB25 RS-485 TERM 2 SA AC Powered
371 10Base-T RJ-45 Fiber ST 3 SA AC Powered
372 100Base-T RJ-45 Fiber ST 2 SA AC Powered
373 10Base-T RJ-45 Fiber ST 2 SA 10 Base FL compatible
374 10Base-T RJ-45 Fiber Singlemode 2 SA 10 Base FL compatible
8321 RS-485 Term RS-485 Term 2 DIN Opto Isolated
8322 RS-232 Term RS-485 Term 2 DIN Opto Isolated
8323 RS-232 Term RS-422 Term 2 DIN Opto Isolated



As we note in the next section, the common situation encountered will be where the premises data communications system is implemented with a single media type, usually Unshielded Twisted Pair (UTP) cable. However, you may come upon implementation problems where the system uses mixed media, some combination of UTP, coaxial cable and fiber optic cable. Mixed media usually comes about when the system evolves and specific segments of it may have to operate in a harsh environment such as a factory floor. The UTP with which the premises data communications system began its growth may not provide sufficient protection in such an environment from electromagnetic interference. The better shielding available with either Shielded Twisted Pair (STP), coaxial cable or fiber optic cable may be required.

In implementing such systems you may be faced with converting the signal modes propagating on one media type so that they are appropriate for another. Adapters for such conversions are readily available.


A variant on the problem of interface conversion is that of the plug-a-verter. Consider two data equipment units, again a computer and a printer, both of which have an EIA-232C interface. In this situation there is really no need to do interface conversion. Most people do not realize that data processing systems such as this often use the X-ON/X-OFF characters to control the flow of data. These systems only need three wires to transfer data between the two data devices. They do not need all of the control signals; that is, all of the pins provided by the EIA-232C interface and its bulky and expensive cable. The data can be transferred from the EIA-232C interface using a modular wiring adapter or Plug-A-Verter and skinny modular cable to connect the three wires as shown in Figure 7.

Figure 7: Plug-A-Verter

A Plug-A-Verter is simply a DB-25 to RJ-11 adapter. This is not as expensive and takes up much less space. If the two data equipment units require a more complicated handshake or control signal, for example a handshake in one direction, then you can go to the next level of complexity using four wires with a Plug-A-Verter. Using the example of the printer, the printer usually has a busy line which would be the fourth wire.

Another level of complexity is the situation where the data processing system needs a bi-directional handshake. This can be accomplished with a Plug-A-Verter with six wires. All of these Plug-A-Verter schemes are more attractive than using the bulky EIA-232C cable with the full interface. More complex systems can use eight wires. As a final point, note that Plug-A-Verters with the associated modular interfaces have all kinds of modular support equipment available on the market. These include wall plugs, wiring closets and multiple port harmonicas.


Given that you have to carry out an interface conversion of a certain type there are a number of factors which may influence your choice of a particular converter model. Below are some of these factors although the discussion is not all inclusive.

Chief among these is whether you want the conversion done by an external box, a card in a rack or a card which fits into a slot on the backplane of a computer. The most common converters are external boxes. But, cards are also available for certain situations. A card fitting into a slot on the backplane has the advantage of being able to use the computerís electrical power source as its own.

You may also be concerned about the effects caused by ground loops and look for a converter which has optical isolation.

Finally, you may need a converter with surge suppression built into it or which has Tempest protection for a military application where low emissions are needed for security.


We have mentioned that the self contained work station usually has within it all of its data communication equipment needs. Nonetheless, there are cases where even these self contained work stations need interface converters.

One situation in which this often occurs is when some military agency procures, commercially available, off the shelf, work stations with EIA-232C interfaces. An unexpected need arises to output data from these units to some equipment using one of the military standard interfaces such as MIL-STD-188-144 Type 1. An interface converter then is obviously required.

Another case develops when a work station with only a parallel output (i.e. Centronics) needs to connect to a serial input device (e.g. a plotter) or when one with a serial output needs to connect to a parallel input printer. A bi-directional parallel/serial converter-buffer is then needed. This is an interface converter.

Please make a selection.