Network Management Information Code
What it is
Purpose of a 0800 network-management message — commonly 001 sign-on, 002 sign-off, 301 echo test; key-exchange and cutover codes vary by network.
Network management (0800/0810) messages are the heartbeat of an ISO 8583 link. DE 70 says why the message was sent: 001 sign-on, 002 sign-off and 301 echo test are near-universal; key change (a new working MAC/PIN key) is 101 on some hosts and 161/162 on others, and cutover is commonly 201. Hosts answer with 0810 and DE 39 = 00. Check the code table of the interface you connect to.
Example values
001Sign-on request.301Echo test (keep-alive).DE 70 on the wire
Bitmap position
DE 70 is bit 6 of the secondary bitmap: byte 1, mask 0x04. A secondary bitmap with only DE 70 present reads 0400000000000000 in hex. The secondary bitmap is only sent when bit 1 of the primary bitmap is set, so any message carrying DE 70 has a primary bitmap whose first hex digit is 8 or higher.
Length
DE 70 is fixed length: always 3 digits, with no length prefix. A parser reads exactly that many and moves on, so shorter values are right-aligned and padded with leading zeros.
Size in the message
At most 3 bytes when the message travels as text (ASCII or EBCDIC), or 2 bytes in binary dialects that pack digits as BCD.
Format n
Format n means numeric: the digits 0–9 only.
Specification reference
Defined in ISO 8583:1987 (base). Card networks publish their own dialect tables (Visa BASE I / V.I.P., Mastercard CIS) that refine formats, lengths, and — for private-use elements — the entire layout. Always confirm against the interface specification you implement.