Telecommunications - X780 Remote Job Entry Protocol

X780 Protocol is an old IBM protocol used for batch processing applications, and is supported over PSS via the Telenet TP3006 configured as RPAD (Telenet use the term BPAD or X780 DSP PAD). This booklet describes:

1 the main elements of X780 protocol

2 the implementation RPAD uses to carry X780 over X25.

X780 RJE Protocol

X780 is carried over BISYNC lines and is used only in a point-to-point environment. The host and terminal ends must 'bid' for the line, by sending an ENQ character. The response should be an ACK0 if the other end will accept the bid.


If both parties bid for the line at exactly the same time, then there is contention, and the 'primary' end (previously defined) will retry after 1 second. If it chooses not to do so, then the 'secondary' is free to bid after 3 seconds.

HOST (Designated primary) TERM (secondary)

Once a dialogue has been established, the end which has established the call will send data in message blocks in a format recognised by the other end, the data control using BISYNC. Refer to the booklet IBM-BSC.TXT for more details.

X780 is used mostly for batch processing applications, where there is signficantly more data traffic flowing in one direction than the other.


carrying access an x.25 network

The software for the host and terminal TP3006's is identical, and so the same configuration is used on both ends. (BPAD and SPAD use different software configurations for host and terminal).

Manual Connect


Manual Connect

The manual connect user establishes a virtual connection, by entering parameters in the command mode.

All command mode directives must be made BEFORE message blocks are being transmitted or AFTER the EOT.

The following commands are directed to the RPAD using manual-connect, are not part of X780, and are not carried over PSS. They are all preceded by a '@' character:

@CONN <sp><address> Make a call to specified

<sp><mnemonic address> NUA

@DISC Terminate call

@SET <parameter +><:><value> Set RPAD parameters

@SET? <parameter +><:><value> Set and read parameters

@PAR? <parameter +><,>------- Read parameters

Reverse Password

Host specific charge? required?

------------- ------------------------

@ID <sp><address> yes yes

@ID <sp><.><address> yes no

@ID <sp><;><address> no yes

@ID <sp><address></><name> yes no

@ID <sp><;><address></><name> no yes

@ID <sp><address></><name> yes yes

Non-host specific


@ID <sp><name> no yes

@ID <sp><.><name> yes no

@ID <sp></><name> yes yes

@ID <sp><;><name> no yes

The @ID command does not support mnemonic addressing.



A user with the auto-connect option has access to only one DTE. The NUA of the DTE is configured into the RPAD, and the connection is made when one end bids for the line.


X25 Call Request Packet


When RPAD receives a line bid, it send a call request packet to the remote end; the call user data field is configured as below:

Byte Bit

0 1 2 3 4 5 6 7


0 | P R O T O C O L |


1 | TERMINAL TYPE (3780, 2780, 2740, 2770, ?) |


2 | S P A R E |





Once a call is accepted, a series of message blocks is transmitted in one direction, whilst acknowledgements are sent in the other direction.


A T :::::: data :::::: T C C


RPAD checks the message for a bad BCC error, and if the message is good, strips off the BCC and PAD (sync) characters before sending them in an information packet. If the data block is longer than the PSS pcket size then it is subdivided into smaller blocks, and each of these is sent as an information packet with the M bit set. The following examples illustrate how message blocks are modified and carried over PSS:

Local and End-to-End acknowledgements


If a message block is terminated with ETB, it is acknowledged locally; (Note: a message block terminated with ITB is not acknowledged, in accordance with BSC protocol).

If it is terminated with ETX (either a single message or the last message block or in a sequence of ETB or ITB-terminated message blocks) an end-to-end acknowledgement is sent.

Error Handling


RPAD follows the normal error handling procedures used in BSC on

the DTE to TP3 port.

The block-check characters are not required over PSS, because it

already features error correction. The remote RPAD reconstructs

the BCC, and reassembles the X780 message block for output to the

remote DTE.




metadata block
see also:


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