PC to PC cable connections

 

 

 

 

 

Cable connections between two PC`s can be used when the PC`s are located quite close together.

The connections can be be in two forms, either serial or parallel.

 

Parallel connection

In a parallel connection there are 8 parallel data lines, each carrying one bit of an eight bit byte.

There are also a number of other parallel lines, carrying control information.

The connectors used are 25 pin D-type, and are called PRN, LPT1, LPT2, or LPT3.

These parallel connectors have changed their characteristics as PC technology has developed -

  • Early PC`s had parallel ports that were for data output only - these are not usable for PC to PC interconnection.

  • Later PC`s had "nibble mode" ports - these had four, rather slow data input lines.

  • Still later came "byte mode" ports - these were bidirectional, using the CPU to perform all the data transfers.

  • The Enhanced Parallel Port, or EPP, were developed, and did not use the CPU for data transfers, control was performed by dedicated chips. Speeds up to 2 Mbps were possible. EPP ports are backward compatible with older port types.

  • The most recent type of port is the Enhanced Capability Port, or ECP - these have added features such DMA, RLE, and data compression. Windows 95 supports ECP.

Parallel connections are intrinsically capable of high speed data transfers, since one bit cycle transfers a whole 8 bit byte.

However the signals are prone to crosstalk between adjacent conductors, giving false information.

Also, on longer cable runs there is a possibility of the 8 bits arriving at different times due to the different cable velocities of each conductor. This is known as skew, and again can cause false data.

For these two reasons, parallel connections are generally limited to quite short distances - say 2 metres, but this is dependent on data rate, and signal rise and fall times.

 

Serial connection

In a serial connection there is a single data channel in each direction - to transmit an 8 bit byte therefore takes 8 bit cycles.

There are also a number of control channels in each direction, in parallel with the data channel.

Serial based PC to PC connections use the RS 232 data comms standard, and use ports on the PC`s known as COM1, COM2, COM3, or COM4.

When used for PC to PC interconnection, RS 232 requires a particular type of cable configuration be used - this is known as a null modem cable, which is essentially a set of crossed connections -

PC 1

 

PC 2

TXD

to

RXD

TXD

to

RXD

RTS

to

CTS

CTS

to

RTS

DSR

to

DTR

DTR

to

DSR

On this type of connection, the circuits are arranged for hardware handshaking using the circuits RTS, CTS, DSR, DTR.

Some computers may require less than this number of interconnects, and some may require more - it depends on the computers, and whether hardware or software handshaking is used. It is possible to do a PC to PC interconnection using just three signals, TXD 1, TXD 2, and signal ground.

Serial PC to PC connections allow greater distances to be covered compared to parallel connections, but are substantially slower.

Finally, it should be noted that the type of cable connection used also depends on the application package being used - normally the same package has to be used on both PC`s. So for example -

  • DOS utilities Interlink / Interserver support both serial and parallel connections.

  • Laplink suppports both serial and parallel connections.

  • pcAnywhere only supports serial connections.

 

© 1998 Ron Turner

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