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Ian Friskney, BAS - Toronto, August 1987 RS232c Primer RS232c PRIMER ------------- One of the more mysterious elements of our technical lives is the workings of the RS232c communications interface. I will attempt to clarify the operation of the subset of the RS232c interface most commonly used. The most common application, today, of an RS232c communications interface is to connect some sort of terminal to a computer. When the interface was originally specified this was usually done via a modem over telephone lines. At the time, computers were large mainframes in central locations. All terminals were remote. The interface specification was therefore developed to standardize the interface between a terminal and a modem. The terminology of the specification and the logic of the control signals is from the perspective of the terminal. The simplest type of terminal to interface is an asynchronous terminal. This is a terminal which does not require a clock signal from the modem in order to interpret the data being received. Asynchronous operation is possible with low speed data transmissions and single level data encoding schemes. Timing of the data is accomplished in an open loop fashion based on the known bit rate of the encoded data and the detection of a start bit which establishes a reference data transition. The data transmission is ended with a stop bit. With ASCII, the data encoding scheme we most commonly use, each data transmission contains eight bits, or one byte, of data. The following information will discuss the RS232c interface only as it pertains to asynchronous operation. EIA RS232c ---------- The Electronic Industries Association (EIA) interface standard RS232c defines the electrical, control handshaking, transmission speed, signal rise time, and impedance parameters of the inter- face between a Data Terminal Equipment (DTE) and a Data Commun- ications Equipment (DCE). The specification is not concerned with the format or content of the data. Nor is the specification concerned with control of one DTE by another DTE. The types of equipment which are typically configured as DTEs include video display terminals, printers, and computers. The - 1 - Ian Friskney, BAS - Toronto, August 1987 RS232c Primer types of equipment which are DCEs include modems, multiplexers, and switchers. ELECTRICAL INTERFACE -------------------- The RS232c interface is a digital interface. Information is represented by a signal being in one of two states. The voltage levels used for the interface are: +3V to +25V is logic 0 or SPACE (High) -3V to -25V is logic 1 or MARK (Low) The signal levels typically used are +12V for SPACE and -12V for MARK. The terminology of the interface is inverted from that of the digital logic we are more accustomed to. That is; if a data bit or control signal is set "on" in the digital logic, then the voltage level at the RS232c interface will go to SPACE. In an effort to minimize confusion, I will refer to signals as being "High" or "Low". A voltage level between -3V and +3V is considered to be undefined or "floating". For a signal to be considered to have switched from say, Low to High, the voltage level of the signal must have become greater than +3V. If the voltage went from -12V to 0V the signal would still be at a Low condition. A signal line which is an output is held Low when not active. This is true for both control lines and data lines. A signal line which is an input is allowed to float and would measure 0V if metered without an external input connected to it. PHYSICAL CONNECTION ------------------- The physical connection of the DTE to the DCE is made through a 25 pin D subminiature connector. Usually known as simply a DB-25 connector. The connector on a DTE should be a male connector or plug; the connector on a DCE should be a female connector or receptacle. The specification recommends a maximum cable length between DTE and DCE of 50 feet. The limiting factor is the total load capac- itance of cable and input circuits of the interface. The maximum shunt capacitance is specified as 2500 pF. Special low capac- itance cables are available which effectively increase the max- - 2 - Ian Friskney, BAS - Toronto, August 1987 RS232c Primer imum cable length. A typical claim made for these cables is a maximum cable length of 500 feet at 9,600 bps. CONTROL HANDSHAKING ------------------- As mentioned earlier, the logic of the control signals is based on the interfacing of a terminal to a modem. That is; the connec- tion of DTE to DCE. To fully implement the asynchronous interface requires eight signal lines. When a DTE is directly connected to a DCE there is a one-to-one correspondence of the signal lines. The following diagram illustrates the relationship: DTE DCE TD 2 -->----------------------------->-- 2 TD RD 3 --<-----------------------------<-- 3 RD RTS 4 -->----------------------------->-- 4 RTS CTS 5 --<-----------------------------<-- 5 CTS DSR 6 --<-----------------------------<-- 6 DSR SG 7 ----------------------------------- 7 SG DCD 8 --<-----------------------------<-- 8 DCD DTR 20 -->----------------------------->-- 20 DTR INTERFACING DTE TO DCE The numbers indicate the standard pin assignments. The arrow heads indicate which pins are outputs and which are inputs. The key to understanding the relationship of the signals is to note that, while the pin numbers and designations are identical on the DTE and DCE, the signal direction (i.e. whether it is an input or an output) reverses. The pin designations are mnemonics whose meaning is derived from the original usage of RS232 as an interface between a terminal and a modem. Transmit Data (TD) - Serial data stream originating from the DTE, being passed to the DCE for transmission. Receive Data (RD) - Serial data stream received by the DCE, being passed to the DTE. Request-To-Send (RTS) - Control signal originating from the DTE. The DTE "requests" permission to begin sending serial data. When RTS goes High, the DCE begins sending a carrier - 3 - Ian Friskney, BAS - Toronto, August 1987 RS232c Primer signal on the transmission line. At the same time, a CTS timer in the DCE is initiated. The CTS timer allows time for the carrier signal to propagate along the transmission path and stabilize before data is modulated onto the carrier. RTS must remain High until the data transmission is completed. When the DTE drops RTS, the DCE will stop sending carrier. Clear-To-Send (CTS) - Control signal originating from the DCE. When the CTS timer expires, the DCE sets CTS high to signal "permission granted" to the DTE to send data. The DTE will send data only while CTS is High. Data-Set-Ready (DSR) - Control signal originating from the DCE. This is a signal indicating the status of the DCE itself. DSR must be High for the DTE to initiate a transmission with RTS or respond to the DCD signal. Signal Ground (SG) - Control signal common between DTE and DCE. Data-Carrier-Detect (DCD) - Control signal originating from the DCE. When the DCE detects a carrier signal on the transmission line a Carrier-Turnon-Delay timer is initiated. The timer acts as a filter of spurious carrier signals. When the timer expires the DCD line is raised High. This signals the DTE that data is on its way. With the presence of DCD the DTE will "look" at the RD line for data. DCD must remain High for the DTE to accept data. Data-Terminal-Ready (DTR) - Control signal originating from the DTE. This is a signal indicating the status of the DTE itself. DTR must be High for the DCE to respond to the RTS signal. - 4 - Ian Friskney, BAS - Toronto, August 1987 RS232c Primer _____________________________________________________________ DSR | __| _____________________________________________________________ DTR | __| ____________________________ RTS | | __| |________________________________ ________________________ CTS | | ______| |________________________________ _ ___ ___ TD | | | | | | ____________| |_| |_____| |__________________________________ | |lsb msb | | | 8 bits Data | | > Start Bit > Stop Bit TRANSMIT DATA TIMING ______________________________________________________________ DSR | __| ______________________________________________________________ DTR | __| _________________________ DCD | | ______| |________________________________ _ ___ ___ RD | | | | | | ____________| |_| |_____| |__________________________________ | |lsb msb | | | 8 Bits Data | | > Start Bit > Stop Bit RECEIVE DATA TIMING - 5 - Ian Friskney, BAS - Toronto, August 1987 RS232c Primer CONNECTING DTE TO DTE --------------------- A common application of the RS232c interface is to interconnect two DTEs without benefit of modems. In order to successfully do this, one must bear in mind the meaning of the signal lines. The first point to note is that the transmitted data of one must become the received data of the other, and vice versa. The second point is that the handshaking logic must be satisfied. The fol- lowing diagram illustrates the appropriate cross connections. DTE DTE TD 2 -->----------------------------->-- 3 RD RD 3 --<-----------------------------<-- 2 TD RTS 4 -->--+-------------------------->-- 8 DCD CTS 5 --<--+ DSR 6 --<-----------------------------<-- 20 DTR SG 7 ----------------------------------- 7 SG DCD 8 --<-------------------------+---<-- 4 RTS +--->-- 5 CTS TR 20 -->----------------------------->-- 6 DSR DTE TO DTE CONNECTION The DTE on the right has had its pin numbers reordered in order to simplify the appearance of the drawing. This cable configuration is known variously as a "Null Modem" or a "Cross Connect Cable". CONNECTING DCE TO DCE --------------------- Connecting DCE directly to DCE is a less common arrangement than the other two configurations. The same rules apply as for the DTE to DTE configuration. The following diagram illustrates the appropriate cross connections. - 6 - Ian Friskney, BAS - Toronto, August 1987 RS232c Primer DCE DCE TD 2 --<-----------------------------<-- 3 RD RD 3 -->----------------------------->-- 2 TD RTS 4 --<-----------------------------<-- 8 DCD CTS 5 -->- N.C. DSR 6 -->----------------------------->-- 20 DTR SG 7 ----------------------------------- 7 SG DCD 8 -->----------------------------->-- 4 RTS N.C. -<-- 5 CTS DTR 20 --<-----------------------------<-- 6 DSR DCE TO DCE CONNECTION The DCE on the right has its pin numbers reordered in order to simplify the appearance of the drawing. FLOW CONTROL ------------ It must be understood that the control signal handshaking orig- inated to regulate the usage of a half-duplex transmission med- ium. That is the lowest common denominator of the communications world. A half-duplex medium is one on which a carrier can be sent in only one direction at a time. The modems are normally in a "listening" mode and can only send out a carrier if no other carrier exists on the transmission line. The hand shaking is primarily concerned with controlling, or signalling the presence of, the carrier. This carrier control is also required in a multi-drop application where more than two modems must share the same transmission line. The handshaking does not provide end-to- end flow control between the DTE. End-to-end flow control is the responsibility of the DTE data protocol. This is outside the definition of RS232c but does require some discussion here. End-to-end flow control is generally accomplished with the Xon/Xoff protocol. These are specific ASCII characters which are transmitted as part of the data stream between DTE. DTE which use Xon/Xoff must have input data buffers. Data is placed in the input buffer at the rate of the communications link. If the DTE cannot process the data as fast as it enters the buffer, the buffer will begin to fill up. When the buffer reaches a certain percentage of full, the DTE must temporarily stop receiving data. It will send the Xoff character to the sending DTE. The Xoff is sent before the input buffer is 100% full because there will be a significant delay before the Xoff can be received and processed - 7 - Ian Friskney, BAS - Toronto, August 1987 RS232c Primer at the sending DTE. In the mean time the data transmissions will have continued. The sending DTE will suspend transmissions until an Xon character is received. When the input buffer is emptied the receiving DTE will send an Xon. OTHER COMMUNICATIONS PARAMETERS ------------------------------- There are several other parameters which are encountered when configuring a RS232c connection, which are not part of the RS232 specification. These parameters deal with the data protocol being used by the DTE and DCE and terminal and modem setup. Bit Rate -------- The bit rates (frequently called baud rate) of a pair of inter- connected equipment must be the same. The bit rate of the RS232 port is independent of, and has no direct relationship to, other timing parameters in a piece of equipment or a system. A common problem is encountered with printers where the printer is capable of printing at, for example, 30 characters per second (cps) but the RS232 interface has been set at 9600 bits per second (960 cps). All goes well until the input buffer overflows; at which point, if Xon/Xoff flow control is not implemented, the incoming data is thrown away. The point to remember is if end-to-end flow control is not available, the bit rate must be compatible with the slowest element of the system. Number of Data Bits ------------------- The number of data bits per character used by a pair of inter- connected equipment must be the same. With ASCII data the number of data bits is either 7 bits or 8 bits. The standard ASCII table requires only 7 bits to represent all its charac- ters. The most significant bit, the eighth bit, is used for a parity check of the first seven. When an extended group of ASCII characters are used the eighth bit is used to provide an additional 128 characters. The extended ASCII characters are generally unique to a particular manufacturer's equipment. Control codes and Function buttons typically generate extended ASCII codes. When the eighth bit is used for data there is no parity check of the data bits. - 8 - Ian Friskney, BAS - Toronto, August 1987 RS232c Primer Odd or Even Parity ------------------ The parity of a pair of inter-connected equipment must be the same. The parity bit is the eighth data bit. It is only used if 7 bits of data has been specified. The parity bit is set or reset to give the total 8 bit data byte either an odd or even bit count. For example if even parity is chosen, the data "3C" would be transmitted as "3C". The data "1C" would be trans- mitted as "9C". The most significant bit, bit eight, would be set to give the data byte an even bit count. If eight bits data has been chosen, the odd/even parity choice is a "don't care". Number of Stop Bits ------------------- The number of stop bits will normally be one. The choice is a throwback to earlier mechanical DTE such as teletypes which required two stop bits. Modern electronic equipment almost invariably uses one stop bit. Xon/Xoff Flow-Control --------------------- If available, this parameter should be selected. This is the preferred method of accomplishing flow-control. For flow con- trol to work, both DTE of a pair of DTE must have Xon/Xoff enabled. Modem Timers - Leased Line -------------------------- There are three timers likely to be encountered in a leased line modem. The Clear-To-Send timer determines the delay bet- ween when a modem receives the RTS signal and begins transmit- ting carrier, until it signals CTS to the DTE. This timer value must be sufficient to allow the carrier signal time to prop- agate to the remote DCE and for the remote DCE to raise its DCD signal. The Carrier Detect Delay timer determines the delay between when a carrier is first received and when the DCD signal is raised. The timer value is chosen to give sufficient time for the carrier signal to stabilize. The timer also acts as a filter of spurious signals. The CTS timer of the transmitting modem must be of longer duration then the DCD delay timer in the receiving modem. The Carrier Turnoff Delay timer is used to maintain a carrier signal on the transmission line for a short period of time - 9 - Ian Friskney, BAS - Toronto, August 1987 RS232c Primer after the RTS signal goes low. This delay allows time for the data to complete propagating to the remote modem. Modem Timers - Dial-Up ---------------------- There are three timers of particular significance in dial-up modems. The Connect Delay timer is initiated when a modem completes dialing a number. The delay is the maximum amount of time the modem will wait for an answering carrier from a remote modem. This value is typically defaulted to 30 seconds. This timer often must be made larger if long distance connections are being attempted. The Carrier Detect Delay timer determines how long a carrier signal must be present before the receiving modem will recog- nize it. This timer is initiated each time a carrier signal is detected. A typical default value is 600 ms., which is normally sufficient. The Carrier Loss Delay timer is used to detect a loss of car- rier from a remote modem. When the timer expires the local modem is hung up. The timer allows short duration carrier dropouts to occur without initiating a disconnect. The value of this timer must be greater then the Carrier Detect Delay timer. Baud Rate --------- Baud rate is a measure of transmission rate. It is used to spec- ify the signal rate on a transmission line. It is frequently mistakenly used to refer to the data rate of the RS232 interface. The data rate is measured in bits per second. A baud is a signal element which can be encoded with one or more bits. A transmission line's baud rate is limited by its bandwidth. The typical transmission line is a voice grade telephone line with a bandwidth of approximately 2700 Hz. This puts a practical limit, of approximately 2000 Bd., on the baud rate. If a modem uses a modulation scheme which produces a one to one equivalency in bps and baud, the RS232 data rate will be limited to 2000 bps. One such scheme is Frequency Shift Keying (FSK). In this scheme, the two different data levels are represented by two different fre- quencies. The lower frequency must be at least equal to the data rate. This is the least expensive modulation scheme to implement. This is the reason for 1200 baud being the most commonly used telephone data rate. In order to transmit at higher data rates, each element of the transmission signal must be encoded with more than one bit. As an - 10 - Ian Friskney, BAS - Toronto, August 1987 RS232c Primer example the most common encoding method used to transmit data at 2400 bps is Phase Shift Keying (PSK). In this scheme the data is sampled two bits at a time. The samples are known as dibits. There are four possible dibits; 00, 01, 10, 11. The dibits are applied to a phase shifter which changes the phase angle of the carrier signal by one of four possible phase shifts. The phase shifts are 90 degrees apart. The result is a complex-frequency signal of 1200 baud. WHEN RS232 IS NOT RS232 ----------------------- The main stumbling block to successfully interfacing to devices equipped with RS232 ports, is the variety of ways the ports may be implemented while still claiming to be RS232. Armed with an understanding of the RS232 specification, it is sometimes pos- sible to work around the limitations of a modified RS232 port. The following table gives some common corruptions of the RS232 specification and possible workarounds when connecting to a fully implemented RS232 port. MODIFICATION LIMITATION or WORK-AROUND ---------------------------------------------------------------- Female type D connector on Make up cable with male con- DTE. nector. 9 pin D connector. Make up cable (or use adapter cable) with 9 pin connector. Non-standard pinout. Make up cable with appro- priate pinout. A terminal, CPU, printer, When connecting a modem to etc. with its port configured this device, use a cable as DCE. normally intended to inter- connect two DCEs. When connecting another DTE to this device, use a one-to- one cable. A DTE whose RTS line is This terminal cannot be used always high. with DCE requiring a switched RTS. Such DCEs are half- duplex modems, multi-drop modems, and port switchers. A DTE whose DTR line is Not normally a problem. always high. - 11 - Ian Friskney, BAS - Toronto, August 1987 RS232c Primer A DTE whose DTR line is not Jumper DSR to DTR on the DCE. present. A DTE which is not controlled Data from this terminal will by CTS. i.e. CTS will be be lost if the DCE is not assumed to be high all the ready to transmit data as time. soon as RTS is raised. A DTE which ignores the DSR The DTE will not be able to line. detect the readiness of the DCE. A DTE which is not controlled The DTE may receive spurious by DCD. i.e. DCD will be data on the RD line. The DTE assumed to be high all the may not be able to detect a time. connect with dial-up modems. Interface uses 0V as Low Will not work with a port signal and 5V as High signal. configured to meet the RS232c specification. A DCE whose DSR signal is The DTE will not be able to high all the time. detect the status of the DCE. A DCE whose DCD signal is The DTE will not be able to high all the time. detect a connect condition on a dial-up modem. A DCE whose DSR signal is not The DTE will not respond to present. the DCE. Jumper DTR to DSR at the DTE to satisfy the DTE. A DCE whose CTS signal is not A problem with command mode present or is low. of some dial-up modems. Jumper CTS to RTS at the DTE to satisfy the DTE. - 12 -