Applies To:
  • CitectSCADA 1.00 1.01 1.10 1.11 1.20 2.00 2.01

The following is some useful information on serial communication, RS232 and RS422. This includes transmission distances for both. 

DTE vs. DCE:

Serial communications devices can be divided into two broad categories: DTE (Data Termination Equipment) and DCE (data Circuit Terminating Equipment). DTE devices are terminals, teletypes, printers, PC COM ports and DigiCHANNEL serial communications boards (to name a few). DCE devices are modems, multiplexers and other devices that connect directly to a data circuit (such as a telephone line). The connectors of these basic device types are configured so that all outputs of one device correspond directly to inputs of the other device, and vice versa. Thus, on a standard DB-25 connector, pin 2 is TxD, Transmitted Data regardless of whether the device is DTE or DCE. The difference is that TxD is an output from a DTE device, and an input to a DCE device. This allows DTE and DCE devices to be connected together with a simple "one-to-one" cable. The following table lists the RS-232 signals supported by DigiBoard for asynchronous communication, and their direction (input or output) for both DTE and DCE applications.

RS-232 Serial Interface

Signal Description DTE Use DCE Use DB-25 Pin DB-9 Pin RJ-45 Pin
GND Chassis Ground N/A N/A Shell Shell 4
TxD Transmitted Data Output Input 2 3 5
RxD Received Data Input Output 3 2 6
RTS Request to Send Output Input 4 7 3
CTS Clear to Send Input Output 5 8 8
DSR Data Set Ready Input Output 6 6 2
SG Signal Ground reference reference 7 5 7
DCD Data Carrier Detect Input Output 8 1 10
DTR Data Terminal Ready Output Input 20 4 9
RI Ring Indicator Input Output 22 9 1


RS-232 serial interface cables should be shielded, low-capacitance cables, ideally designed specifically for serial data transmission.


The shield should be grounded at one end of the cable. Chassis ground--available on the shell of DigiBoard's DB-25 and DB-9 connectors, and pin 4 of the 10-pin RJ-45 connector, is ideal for this purpose.


While good shielding provides reasonable protection against "noise" (Electro-Magnetic Interference, or EMI), cables should still be routed away from noise sources wherever possible. Avoid laying cables in close proximity to transformers, generators, motors, fluorescent lights, etc. In very noisy environments, it may be advisable to use an RS-422 interface instead of RS-232. PC/16i boards are available with RS-422 hardware. which is much less susceptible to noise, and can handle cable runs of up to 1200 meters. NOTE--RS-422 and RS-232 interfaces are not compatible. If you use an RS-422 interface, the hardware at both ends of the cable must have an RS-422 interface. RS-232 to RS-422 converter boxes are available from suppliers such as Black Box.

Capacitance vs. Length of Run

The total capacitance of a cable affects the integrity of transmitted data. As a rule of thumb, the total capacitance of a cable (including the connectors) should not exceed 2500 pF. Serial interface cable is usually rated in pico Farads per meter. Therefore, if a cable has a capacitance of 165 pF/m, and the connectors are 100 pF each, the maximum recommended cable length is 14 meters. If the cable is rated at 40 pF/m, the maximum recommended cable length is 57 meters, and 16 pF/m cable can be run up to 143 meters. In situations where low-capacitance cable is unavailable, or very long cable runs are required, "short-haul" modems, available from suppliers such as Black Box, can be used to increase the effective range of the RS-232 interface. Short-haul modems are similar to standard modems, except that they are connected directly to each other via a cable instead of going through a telephone circuit. NOTE: Use only externally-powered short-haul modems with DigiBoard products. Another way to achieve very long cable runs is to use an RS-422 interface instead of RS-232. PC/16i boards can be ordered with RS-422 interfaces, and suppliers like Black Box sell RS-422 to RS-232 converters which can be used with peripherals that do not have RS-422 capability. The RS-422 interface has a range of up to 1200 meters.

Signal Description DB-9 Pin
TxD+ Transmitted Data (+) 8
TxD- Transmitted Data (-) 9
RxD+ Received Data (+) 6
RxD- Received Data (-) 7
RTS+ Request To Send (+) 2
RTS- Request To Send (-) 3
CTS+ Clear To Send (+) 4
CTS- Clear To Send (-) 5
GND Chassis Ground Shell

The RS-422 version of the PC/16i Digiboard provides asynchronous serial data communication over differential lines. This permits cable lengths much longer than those supported by an RS-232 interface (up to 1200 meters), and has better noise immunity at high baud rates. The PC/16i RS-422 interface supports two input pairs (RxD and CTS), and two output pairs (TxD and RTS).


RS-422 interface cables should be shielded twisted-pair cables. Each signal requires two leads (one twisted pair of wires) to complete a balanced voltage digital circuit. The shield should be connected to the Chassis Ground of the devices at both ends of the interface cable. Chassis Ground is available on the metal shell and on pin 1 of the Digiboard DB-25 connectors.


To insure the integrity of data transfers and control signals, a ground path must be provided between the devices to be connected via the RS-422 interface. This should be connected to the Chassis Ground of each device. DigiBoard recommends using the cable shield for this purpose.